Why do men (or women) get into relationships w/ people who have kids w/ several other people?

I got into a relationship with someone who had a young daughter once...I wanted to end the relationship a year before I did but did not cos I was so close to his daughter... It breaks my heart everyday  still that I got cut out of her life and she doesn't think I care about her anymore.... I don't think dating someone with children is a good idea then , but I'm a bit jaded.