We had a few years' gap of not really being in touch during undergrad but that changed as we had a few months' overlap at post-grad university. It was easier to meet after that and we did keep in touch, but I felt that he was holding back somewhat and not really being free with his thoughts, letting the conversation flag at times.
So at a friend's wedding a couple of years ago, I opened up to him and plainly told him that it's a shame we don't talk as we used to as we both are clearly on the same wavelength when it comes to shared interests and sense of humour. And this worked - our degree of friendship has increased an order of magnitude since before that time. I would have lost a great friend to the vagaries of life had I not taken that step to become vulnerable for an instant then.