Some researchers think this effect comes from our need to reconcile us doing someone a favor and us not liking that person, so we assume that we like them.
Other researchers think that the one being asked for help senses that the one asking wants to get friendly with them and in turn reciprocates the liking.
Is a psychological phenomenon that causes us to like someone more after we do that person a favor: We justify our actions to ourselves, that we did them a favor because we liked them.
But the reverse effect is also true - we come to hate our victims, which helps to explain wartime atrocities.
Cognitive dissonance makes our mind try to protect our self-image and the connection between our thoughts and actions by modifying our opinions.
Once the justification or new opinion arises, we become more sensitive to any information that supports it. We also get more skeptical of any information that opposes it.