Have you ever watched people wait for an elevator? When it arrives, a startling number of them will start walking in before the doors are even done opening. Quite often, they stop short as they realize someone else needs to walk off first.
I'm guilty of this myself, occasionally. I always rebuke myself and vow not to do it again, but it's a hard impulse to resist. After all, I called the elevator, and it came. It is here for me.
Except that it's not. The elevator often, or even usually, has other things to do than shuttle me around. It has to shuttle other people around too. People I'll never speak with, to floors I'll never visit! The elevator arrived when it did because it was on the way past anyway, or because someone else had already asked to go there. Sometimes it was just sitting there with nothing to do, waiting to be asked the favor of carrying someone, but often not.
People are like elevators. When someone is doing something that coincides with your goals, it's strongly tempting to assume that you have some shared goal or motivation in common. Or even to assume some sort of good will. The reality is that people have their own reasons. Even if they have literally nothing else in this world, they have their reasons. And it's exceedingly unlikely that they coincide with yours. Maybe they even conflict with yours! In that case, it can be quite risky to assume otherwise.
That doesn't mean that we can't happily cooperate or commiserate and then part ways peaceably, having both benefited from sharing the load or the journey. It just means you'll be happier, and less often surprised or put out, if you keep that perspective in mind. We've all got our own reasons. Even you. And yours are just as invisible to everyone else as theirs are to you.