As rand al'thor mentioned, the answer is gravity.
Your heart pumps, asking why
Your heart has to work against gravity to pump blood to your head.
"I am here to make you cry.
It's hard to say goodbye,
But leave me so you could fly."
Without gravity pulling you down to the earth, you could fly.
Your mind said love, but your body said hate
I'm not sure how this line applies.
happiness is together and you gain weight.
Gravity causes things with mass to have weight as well.
Like when we met on our first date.
I'm not sure how this line applies.
Nothing but fruit you have ate.
Sir Isaac Newton discovered gravity while watching an apple fall.
Sounds like a story, romantic,
nothing around me but tragic.
The gravity of a situation, for example.
Here I will show you a magic,
I'll vanish and you will panic!
If gravity were to vanish, nothing could hold itself together anymore, and you would die.
I might be the reason you fall in love,
But love is nothing I planned to speak of.
Gravity is the reason you ever fall at all. Love is irrelevant there, which is what the second line is for.
I am massive and I can be related,
Gravity affects masses, and is a component of general relativity.
Two great minds in history have debated.
Copernicus and Galileo, I assume?