Inertia
Life is a constant battle against inertia. It's easy to keep doing what you're doing because it's what you've always been doing. The routine is addictive. It's predictable. Humans like that.
The problem is life goes by and then you look back and think, "Shit, I haven't done anything I actually want to do."
So you have to constantly fight it. You have to actively make changes. You can start with something as simple as taking a different route home from work. Or stopping by a different grocery store. Or if you're really feeling ambitious start that side project that will help you change to the career you've always wanted.
Inertia is a bitch. Fight it.