The answer to this question is rather straightforward. For many Christians, the substance and justification of their religion is the social life that flourishes in and around their church. Considered in this way, a rejection of Christianity is a rejection of not just bible study groups, but support groups, places presumed safe to meet the opposite sex, friendships, summer bible camp, the list is endless. The social power of Christianity should not be underestimated.
Because of this, a typical believer feels it to be an affront and an implicit attack on all these activities and on the images that swarm around in his feelings for his religion. The atheist, then, is the consummate outsider and such a one cannot share in the believer's hopes, fears, values, and feeling for life.
Doctrinally, although God is love, He is also a punishing God. Intercourse with non-believers carries a risk of contamination and eventual eternal damnation. Consequently, in order to preserve what he holds dear, the Christian fears and hates the nonbeliever. Or rather he seeks to convert, assimilate, and remove the danger.