why are guys so careless?

So me and my boyfriend have a 5 month old baby. We've been together for over 2 years and he's been in love with me for about 4-5 years ... we've talked on and off before we actually started dating ... and yesterday he decides to break up with me just because he simply doesn't have feelings for me anymore ... also he thinks he likes this girl he works with . How do you just wake up one day and stop loving somebody ???? Somebody please explain