Resistance to Loving-Kindness

I have been continuing my meditation practice, and I feel there would be a great benefit from switching to a metta practice instead of breath. Yet, with metta, a strange phenomenon occurs. I am usually very good *during* metta bhavana and generate the feelings. However, it is incredibly hard for me to begin the metta session, and I seem to avoid this type of meditation before and afterwards. Only during the practice do I feel great emotion and compassion. How can this be explained? Thank you.