For me it's simple I just hate to work in companies that are not mine, so when I get home at night my brain keeps telling me my life is going to be a nightmare if I don't do my stuff.
I just can't sleep without commiting some code, or looking for a solution for a problem I am having in my side project, I need to at least search for it.
If I stay too much time ( Girlfriend, family, sports etc..) like 5hrs or + away from my things I start day dreaming about problems I have to solve in periods I am alone but cannot get to a computer, and I feel a urge to get home soon.
Anyway it's my curse... I gotta have side projects otherwise live is meaningless( at least at this time of my life)
It's me and sometimes people get pissed at me but I am like that love me or hate me :D.
P.S: a LIGHT ( not running a 15kms) exercise really helps, it's like your body is ready for the next 24hrs at 9pm.