I was thinking about this recently, also.

There exist many people who I am certain I would consider very good friends - if only I actually saw them more often. People with whom I know I get on well with, or who challenge me by being interestingly different to myself.

You would be a good example of this, I consider you a friend, although I know we're not very friendly very often. This is a bad thing.

I sometimes think I should make more of an effort to see people who I don't meet very often, and then I think I shouldn't have to, if we're friends despite not meeting very often maybe that's just the way it works out. Maybe meeting more would spoil things.
I'm still undecided, although I can recognise it's my lazy side is coming to the fore and suggesting the 'continue not seeing very often' idea.

I can certainly think of a few very good, and close, friends with whom spending a lot of time is a real effort. It reminds me of a time a few years ago when I shared a flat with a particular 'character'. He's lovely. He's kind, thoughtful, bright, interesting, and fun to spend time with - but at the same time I'd never ever want to live with him again.