I think its matter of equal effort being made. To go with the tennis analogy. Sure the first couple times you call your friend to invite them to play, it doesn't matter, but when your the one who is ALWAYS calling and inviting, and the friend never calls to invite you to do things, you start to wonder if the friendship is rather one sided. Its the same with sex. When your the one always initiating sex and your S never does you start to doubt if your S wants you as he/she never makes a move toward you. Everyone wants to be "chased" a bit and feel desired.
