Jim reacted the way he does to Della's short haircut, because the gift he bought for her (hair combs i believe) required long hair, and because it was one of their most 'treasured', and also the old watch (which Jim had sold to buy the hair comb)
I wasn't sure myself, but this seems like the only answer that sounds correct... I really hope this helps you! (If you would like an explanation, I can comment it below) xx
