In the comics, when Thanos was a kid he was torturing bugs or some shit - as psychopaths do - and then a cute girl a few years older than him shows up and starts chatting. She's death, literally, the marvel version of the grim reaper.
Thanos falls in love with her, so he tries to impress Death by killing off everyone on his planet. Death shows up, but instead of being impressed she laughs at him for thinking it would impress her.
So, Thanos decides to kill off half the Universe because he wants to get some of that death pussy.
Unfortunately, Thanos gets friend-zoned because Death has the hots for Deadpool. Awkward.
This movie still sucks dick, btw. You people have no taste.