Their enmity started with a bang, a destroyed Fortress and attempted murder. It seems appropriate, then, that it would end with a slow slide to forgiveness. Clark has watched Lex over the years, and time has revealed the truth that a teenaged Clark was too naïve to really understand. Lex has no one, has never had anyone but Clark, who was ripped away from him in what must have seemed like an exquisite betrayal. Clark has toyed with the edges of that kind of loneliness before, and it's enough to drive anyone mad.
For all that, Clark knows that it won't be the build-up of all these little things that will cause him to take the plunge that seems so inevitable now. If Clark knows one thing about himself, it's his tendency to get caught up and do things in the moment without really thinking. Eventually, he knows, there will be an instant when reaching out to Lex will suddenly crystalize into the right choice, the only one he can make. And that will be that.
Their friendship ended with a slow slide to anger and secrets and lies. It seems appropriate, then, that it begins again with a bang.