Both are correct. If you see 'giving and sharing' as part of the same activity, you can use 'makes'. If you see 'giving' and 'sharing' as two separate activities, you can use 'make'. The choice is yours.

I believe the first one is correct, because it's the fact of giving and sharing that makes you feel good. Like in, "it feels good". It's the way you accord the third form of singluar. I make, you make, he makes. I feel, you feel, he feels. Etc.

Both are correct! If you count things as in a group, then the group is singular and so the verb takes an "s". If you consider the things as distinct items, then they become plural and the verb do not take an "s" after it. The only time even if you consider the disparate things as a group while the majority believes that they cannot be consider as a group, then your grammar is considered wrong by them. For example, "the boy and the girl do not share the same things". Here the boy and the girl cannot be considered as a group.