Noradrenaline

Noradrenaline is stimulating neurotransmitter similar to adrenaline with all its "flight-or-fight" reaction but it is more common in brain where it's spread from 2 tiny blue areas (locus coeruleus) in back of brainstem to all over rest of the nervous system.
Maybe less known attribute is that this substance seems to cause attraction or love. I learned that from personal use of reboxetine that inhibits re-uptake of noradrenaline after it's released. Also amphetamine and cocaine seems to have similar effects and these drugs often increase effects of noradrenaline, adrenaline and dopamine at the same time (these 3 receptors and neurotransmitters have similar structures). 

It seemed to cause increased strength and stamina with push-ups. Sometimes one 4 mg pill doubled the amount i could do before exhaustion.

Sympathetic (blue) and parasympathetic (red) nervous system (NS) are opposing sides of autonomous nervous system. Sympathetic NS (largely activated by adrenaline and noradrenaline) increases energy use, reduces digestion (constipation possible) and activates muscles from skeletal muscles to constricting tiny muscles around blood vessels. Meanwhile parasympathetic NS (mostly activated with acetylcholine) defends against excessive activity caused sympathetic NS almost entirely by vagal nerve that can slow down heart and lung activity. In case of high blood pressure vagal nerve can lower it. Parasympathetic NS also causes sweating and speeds up digestion by releasing more gastric acid and potentially causing diarrhea.
In blood circulation sympathetic NS causes more blood flow to brain and skeletal muscles while constricting blood flow to gastrointestinal tract, genitals (might cause problems (impotence and dryness) during sex) and kidneys. Parasympathetic NS reduces blood flow to brain and skeletal muscles causing drowsiness felt for example by inhaling deeply and slowly exhaling through almost closed mouth. It increases blood flow to genitals, gastrointestinal tract and kidneys.  

In sexual function parasympathetic NS causes orgasms while sympathetic NS causes ejaculation. I rediscovered that last part on my own when i tried masturbating after taking reboxetine. Ejaculation started without any hint of orgasm maybe 5-10 seconds after start while i didn't feel orgasm. It was possible to resume and reach orgasm with ejaculation minute later. 

Noradrenaline is a suspected cause for some of loves symptoms like restlessness, emotionality and lack of appetite. Attractiveness of others depends on noradrenaline and dopamine levels in observer. Sex hormones can increase noradrenaline levels in hypothalamus and their own levels may be increased with noradrenaline. Amphetamines in optimal dose are known to cause attraction.

If someone wonders how to cause attraction with some "love potion" then noradrenaline agonists or re-uptake inhibitors may be closest thing to that. At least reboxetine causes attraction for almost anyone (better looking 50% in random room) but this attraction may pass in more sober state or with all the doubts about having these emotions due to some substances. During some school classes i noticed 5-7 girls that seemed most attractive on different days with same people becoming most appealing during different days.

Reboxetine caused bravery and feelings of power. Body felt strong, rested and extra light. Breathing becomes deep and strong although during withdrawal breathing became disturbingly difficult (adrenaline like substances broaden airways and help against asthma).
After quitting reboxetine it took me about 1-2 weeks to feel normal. During use i felt i was falling in love with almost any at least average looking woman but after quitting it took maybe 1-2 months before i felt attraction to anyone again. In these 30-60 days i felt constantly too tired to feel emotions.
Loud music used to become more pleasant after taking pill but after stopping i had to turn music to minimum for about week because it caused headaches very easily and it felt too tiresome.

High dose (8 mg per day for days) felt like body had huge pressure in limbs and lowering dose created feeling of happy relief. 
It commonly caused tense feeling in back of throat and often some metallic taste (possibly from burst blood vessels). Voice got louder.

Often i felt like noradrenaline was something fanatically behaving people have high amounts. It gave me energetic behavior with wide open eyes and strong drive to do something even if i didn't find something to do.  

Physical exercise gave much faster feeling of runners high that lasted maybe 30 minutes after 30 second physical straining. It also caused faster nausea, headache and tense feeling in neck and back of mouth. 
Music caused goosebumps more easily.
Pain felt more distant.
It gave strong sense of independence but this feeling was replaced by sense of physical weakness and dependance for other people with during below average doses or after quitting.

Reboxetine has a 13 hour half-life but many effects weaken within first hour. Sense of hunger may return about 24 hours later and i could easily go entire day without eating and without hunger although weakness after not eating was common.

Sometimes it made me feel like time had slowed and my reaction times felt faster.

Messing with stress hormones can be unhealthy because these hormones also cause release of cortisol that speed up metabolism but also inhibits immune system. In extreme cases cortisol is used to shut down immune system after body attacks transplanted organ or even during infections. I don't remember being sick more often because of reboxetine but knowing it still helped with quitting that.  
For example some salmon species die after spawning because their immune system didn't work well after 5-7 fold increase in their cortisol levels compared to levels before spawning age. High stress hormones probably helped them swim upstream without hunger but it could also causes death.

According to a study (mostly about knock-out drugs) people sometimes secretly give amphetamines, MDMA or cocaine to make other person more sexually or emotionally receptive.

Usually the formation of attraction went through similar phases. It starts with noticing someone and getting pleasant thoughts about them. With repeated meetings these emotions deepen and get more difficult to ignore. Pleasant fantasies (kissing, hugging, sex etc) about intimacy are usually more pleasant than routine and thoughts start to increasingly revolve around a certain person. Disproportionally pleasant fantasies get more central in daily thoughts due to other joys seeming insignificant and as time goes on it can turn into distracting addiction for some person. Sense of growing together with someone you have known for long time is common if other person didn't cause unpleasant memories but with love it can feel more extreme.
Knowing how these emotions usually go didn't reduce pleasantness of love for me. More likely these experiences gave confidence about knowing how such emotions usually work.

It also caused less monogamous thoughts (not actual polygamy) and less feeling of depending on one person.

Testosterone on human volunteers didn't seem to cause changes in behavior.
Considering how reboxetine affected me i'd say stereotypical "manly" loudness and feelings of strength/power can come from extra noradrenaline. Sex hormones do influence production of noradrenaline in at least hypothalamus (place that can control overall wakefulness). Stimulant users may behave aggressively without any deliberate testosterone consumption. 
If they had less confidence then they'd probably also yell or fight less.