You should measure your levels of oxytocin (along with dopamine and norepinephrine)
Is highly critical in human pair-bonding: it helps to have a "memory" for the good situations and people, so increases when someone makes you feel confident, empathetic, calm. And to release to the stream it is necessary to have a sense of wellbeing and physical contact

So increases your desire for your couple, it creates sexual arousal, and helps to maintain erections in males (

) When you're sexually aroused, oxytocin levels increase in your brain significantly and during the orgasm, the brain is flooded with oxytocin, so... When you cuddle to your girl/boyfriend is the oxytocin acting. The levels of oxytocin tend to be higher during both stressful and socially bonding experiences (a.k.a. romantic relationships haha)
It's basically a brain thing.
You're welcome.