Why would my dad shoot himself on my birthday?

My mom remarried when I was 3. I kept in contact with my father until I was about 12 when my mom and step dad told me and my brother we couldn't see him anymore. This wasn't my choice but by the time I was old enough to make my own choice I had such a distance that I didn't really want a lot of communication. So when I was 16 I ran into at my job and got his contact info. For the next few years we would text off and on. But I was never at a comfort level to talk on the phone or go see him when I was in town. The last text conversation we had was almost a year before he shot himself. He talked about how much life sucked and I told him not to do anything to hurt himself and he told me it wasn't his style. He was in bad health with diabetes and had one leg missing because of it and lived with his dad all his life. So on my 23rd birthday ( I had actually got into town and was planning on going to see him this time and communicate) I was taking a nap on the couch waiting for my family to get home to go have dinner with my grandma who shares the same birthday, and my brother called me and told me what happened. My dad survived a year and 2 weeks exactly before dying of other health problems related to everything else. I never got the closure I needed as to why he did it on my birthday. Was it planned? I know he loved me. I was his only daughter and always tried to forgive him. Sorry that its so long.