[E]verything someone does for you has an opportunity cost. That means if someone takes time out of his or her day to attend to you, there’s something they haven’t done for themselves or for someone else. It’s easy to fool yourself into thinking your request is small. But when someone is busy there are no small requests. They have to stop what they’re doing, focus on your request, and take the time to respond. With that in mind, there is never a time when you shouldn’t thank someone for doing something for you.
Source: What I Wish I Knew When I Was 20 by Tina Seelig