You do not have any spare time. Ever. And neither does anyone else. Keep that in mind when you ask things of people. Anything you ask someone to do must necessitate them not doing something else, and that “something else” was already on the docket.
Opportunity Cost is real, and doubly so for time over any other resource. That doesn’t mean you can’t ever ask someone for a favor without feeling guilty about it. It just means to be respectful of the fact that you’re really asking two favors: you’re asking them to do something, and you’re implicitly asking them not to do something else.