Can someone please explain this to me, i just don't understand how...?

I don't understand how someone says they love someone sooo much & always has & tells everyone how much they love them BUT chooses to be with every other person except the person they say they are in love with. I just don't get that!! Can someone explain that? Bcs i KNOW if i got that chance to be with that one person i know love more than anything i'm GOING to take it & NOONE else could ever do... Ugh please just explain this to me...