Why do men pretend to be so nice and get u to fall in love with them (I am talking men age 38!) .?

Then u fall in lvoe with who u think they are and of course there is and then the calls die down, everything chagnes, they say they cant commit due to they are bust work. So u dont call anymore but they still call you on occasion and want to see you- u see them and there is alot of physical so u are thinking ok they really do want a relationship. but it turns out they dont- they leave you for someone else???? what is with all this? if they just wnat a short term thing ( and they know this is not what i want) w hy waste my precious time ????