Sexy time

Me
So me and my bf had sex three times , and one late night i was falling asleep and he calls me in the middle of the night that he was horny , and so we had phone sex until the morning we had to do it but this time he came fast and his worried amd i was like dont worry i mean thats normal right? 
I mean i really dont mine bc am not a "sex pro" i only hadsex three times with him and only him and it still hurts lol 
But why do men cum fast?