Well, it is because of endorphins. Love is a drug, whenever you see someone you love, whenever you remember memories of your love, it releases endorphins in your body which will give you a high, just like doing drugs. But whenever you cross paths with someone you hate, your subconscious triggers its protective mechanism releasing different hormones cause you have bad memories with them or they exhibit behaviour that makes you uncomfortable. Those hormones make you feel shitty, hence you feel low.
Fun fact: Do you know Oxytocin is responsible for both love and hate? So whenever you love someone technically you are also hating them.
We are wired to avoid pain and seek pleasure. So love always wins in the end as it is a drug and hate always loses as it makes you feel shitty, I mean, come on, nobody wants to feel shitty for a long time. Having said that, when I say "love always wins" I mean "the other person may not love you back but they give up on hating you, hence creating a possibility to fall in love with you but it is highly unlikely"
As much as people love to believe "Love is a strength"
I would like to say "It is a weakness disguised as strength" (But believing in the former makes you feel better, so go ahead, no problemo)
So, love, is a drug and drugs are not good, they will make you lose control and make you do stupid things.
So hey you...................yes I am talking to you, don't do drugs ok, drugs are not good.
and btw we can hate the one we love, after all we are human, but if you truly love that fella, hate is nothing but a simple heartache ( lol almost copied from a song) Thus, Love and Hate --- love always win in the end. 
when all it does is just pump and supply blood 
