I don't understand why?

One of the main reasons that I moved to Terrace bay was to get to know my father better. We went along time without speaking when I was a teen. My Father drove a transport truck for a living and was not home very much. needless to says we never got together from the time I was 16 until about 18. Alot of tings have happened since then that have brought us together. My step-mother Passed away suddenly a few years ago. The following spring my Step-brother lost a long battle with cancer. These two things changed the dynamic between my father and i profoundly. I let go of the anger i had harbored for so long about growing up without him around. I know alot of people have to grow up without a parent but for a teen-age boy it is hard not having someone tell you the decisions you are making will affect you for life. My mother was the only one who was there to do that for me.
I am married and have a new born son. I don't Know why my dad exactly left( everyone's story is different) but i do know that I couldn't leave my son. ever. I think that sometimes people just get fed up and decide that there is nothing left to fight for, but i don't understand why?