It is like Tom Quinn -needs- to justify everything he does, if not to himself then to the people he cares for (puts in danger). In a way it is almost the coward's way out because Tom has a conscience and it tends to waken at the most inappropriate time (everytime he does something that he know is morally wrong), and if he can't forgive himself he'll get someone else too. I think by the time he left MI-5, Tom may have found some form of peace, and I really hope that Christine and him (man, people seem to hate her) did somehow ends up together again.

Except realistically speaking, Christine probably hates his guts on some level.