Because I got nothin’ else at the moment.
There are ten stoplights between my house and work. (I know, it’s a short commute.)
Of those ten, I will always stop at three in particular—no matter when I leave, or how many of the others I have to stop—or don’t stop—at.
Beyond that, it’s a crap shoot. Some days I can breeze through with only four total stops, other days it’s six. On rare occasions I hit only those three lights red, and on very rare occasions I end up stopping at 9 out of 10 (one intersection is always green).
But on average I stop at five lights on my morning commute.
Wasn’t that exciting?