A few days ago I was listening to a recording of a design discussion, and I was at two minds about what was going on in my mind when I said some of the things that I said. I found myself in the odd position of both remembering my reasoning and listening to them as an outsider. It gave me some insight to the way I think. It is not really an analytical process.
There is a lot based on the taste of the idea. I am sorry, I am not sure how to express it in any other way. I decide on the idea/design based on how it feel. There was a lot of that in the discussion, for instance. I can usually explain my initial response when I give it some thought, luckily.
At one point there were about two minutes of failed design attempts, we can do X and then... hm, not that is too complex, so we will do Y which would... no, that doesn't handle the Foo requirement, etc.
It is partly preference, partly experience and partly the appliance of good habits learned from painful experiments.