Because basically love is happy, if in the end you hurt it is not because of love but because of ego. The ego is too strong to have the person you love, while he has had his own happiness with his partner. Even if he finally chooses you and leave his partner, it is also not love if in the end there is a hurt.
Because the essence of love is to accept and live, is not God the best screenwriter, so why remove the best part of Him and replace it with a scenario according to our ego.
Believe that God loves you even if you repeatedly run away from Him. He is the closest place to throw the burden, even though you come only when necessary.
Learn to love who loves you because that's where you'll find happy.