First of all, we try to ask someone to let go of an issue we can’t agree with him on. Ideally, we don’t ask someone to let go of an issue if it won’t let go of him or her, yet. We just listen until he or she works it out or gets stuck in a corner and needs us to pull him or her out of it. Though letting go is lauded, it often involves invalidating someone’s feelings. It is very hard to let go of feelings. It’s different from focusing cognitively on something; it’s visceral–and hurts, so we need to talk it out–to vent, sometimes.
Next, we don’t have to take sides. When someone asks for this he or she is insecure and when he or she expects it, that’s bordering on control. If a person wants your allegiance, he or she is asking for you to surrender your feelings and your mind and your freedom. Strong, healthy people who love one another don’t do this. Weak people in co-dependent relationships do. And, lots of people do it because, 1. they think friends are members of their armies, and, 2. they don’t understand love and friendship, which require people to be free and to be themselves–not what friends and lovers want them to be.
You do nothing wrong by disagreeing with someone. You can say you understand, or that you empathize. You can share in the pain, but to be a friend you shouldn’t fake agreement. This paints a false picture of who you are and will set up misunderstanding and co-dependence.