Asking things personally from people has a way of toiling with emotions at conscience level. This is especially if the person is an acquaintance. Sometimes, it works for random strangers too.
When someone walks up to you and asks you to do something for them as a favour, not doing it will sometimes make one sober. It’s even more sobering if the person asking for the favour is living in the same vicinity.
Apart from that, there is also the fact that needs are something everyone can relate with. Since we all desire things ourselves, who is to say that the same person may not be in a position to help satisfy their own needs tomorrow. There are just too many variables.
From a different perspective, it may also be linked to the fact that humans like satisfying needs and desires. Therefore, knowing that someone else is in need of something, and that it means a lot to them, a certain empathy automatically gets created in our mind and fills the thoughts.
This may not be unconnected to the reason why some people who are indebted to someone starts acting weird, especially when they are dodging payment of their debt. The ways it manifests itself is unending and varies depending on the circumstances.
Most of the times however, when someone asks us for a small favour, not doing anything about it and seeing the same person around creates a feeling of empathy. This mostly arises if the person being requested of the favour is in actual position to do something about it.