I don't know if you're like me, but when I'm pensive, the emphasis is not really on the "thoughtful," it's on the "wistful." I don't really think. I reflect in an awkward way, re-imagining past events as having different outcomes and picturing how myself and my life would be different.
I suppose I actually am thinking. This method is practiced in my head. I suppose the point is that I'm thinking poorly. I don't utilize logic and these scenarios are necessarily outside the realm of reality as they didn't happen. It's my evasion, and I need to figure out how to stop it.
To a certain extent, it probably, mostly, has to do with the way I grew up. I wouldn't call it a necessary evasion (evasion is never necessary as it is always harmful), but I would call it a coping mechanism. The only subject I'm ever like this about is social situations. For a long time, I found it safer, easier, and more rewarding to conceptualize permutations in my head than experience actual instances. In high school, when I wasn't too bad with this, the permutations were fleeting, as work and sports dominated my mind. In undergrad, when I was the worst with this, the permutations were intricate, as I had far too much free time and focused far too little on school work. Grad school featured a return to my fleeting nature. I was focused on the two tasks of earning my degree and learning how to teach. I simply didn't have the mental capacity to worry about more than those things.
Then I graduated and found myself with the luxury and resources, for the first time in my life, to address this issue. Days like this are far less frequent now, but they still happen, especially on Sunday mornings, and that's far too frequent for me. I think the only remedy is a higher dose of reality.
(Btw, the No Doubt song isn't really relevant beyond the title, but whenever I think of Sunday mornings, I always think of it, and I won't pass up an opportunity to promote Tragic-Kingdom-era No Doubt.)