What Makes It So Easy To Walk Away From Children These Days?

Being a single parent with no help from a co-parent, has got to be one of the loneliest things on this planet. No one enters into parenthood believing that they will, solely, be responsible for raising a child. Obviously, I'm not talking about those that opt to have a child "unconventionally" I'm talking about two people who were in a loving relationship, or at the very least, liked each other. I will never understand a parent being absent from their child's life. I don't want to sound judgmental, but it truly is something I can not comprehend. To totally cut your child out of your life, seems so heartless. I'm a single mother of two teenagers and its so difficult for me to deal with the fact that their father does not bother with them. It's such a lonely feeling, when you feel like you have nothing left to give, and there's no comfort from the other parent. My mother has supported me and been there the whole time. It's just not the same, at least to me it's not. It's not her job to be co-parenting with me. She's supposed to be enjoying her grandchildren and spoiling them rotten, then sending them back home to me. Instead she's there, worrying, every step of the way with me. I'm struggling to raise these beautiful children, one of them being a son. How can I give him what he should be getting from his father? It makes me so upset. I never married, but came very close. The last man I was with, for five years, promised that he would always have my son's back. Where is he now? When my relationship ended with him, so did the one he had with my son. He's now playing house with another woman and her children. And I was engaged to him! What makes it so easy to walk away from children these days?