For the most part, people don't mind helping out one another. If you're asking for a favour, however, it never hurts to explain why. In fact, according to a classic study, your reason doesn't even need to be a good one. Photo by me and the sysop.
The study, led by Ellen J. Langer, and published in the Journal of Personality and Social Psychology in 1978, is an "oldie but a goodie". It suggests that people are more likely to do you small favours if you provide some sort of reason why, no matter how obvious the reason is. For example, participants in the study were more likely to let someone cut in line to use the office copier 90 per cent of the time if the cutter had a reason why. Even if the reason was "...because I have to make copies". There are a couple reasons this may work. First, it's likely that people aren't really listening to you. They hear the word "because" and automatically assume you have a good reason, otherwise you wouldn't be asking. Second, it could be out of pity or because they don't feel like arguing about how ridiculous your reason is. Either way, you might as well give a reason when you ask for a favour, no matter what it is.
The mindlessness of ostensibly thoughtful action: The role of "placebic" information in interpersonal interaction. [Journal of Personality and Social Psychology via Science of Us]