I seem to have a certain literary fascination with mountains. Given a choice, I will almost without fail set my writing in a mountainous region.
What attractions do mountains have for writing? We humans prefer to build in valleys and plains, on the banks of rivers and the shores of seas. Down through history, we prefered the trade possibilities of ports and the agricultural promises of flat land. Mountains are the opposite of this. They are intrusions that are difficult to tame. They require adventure and journeying to go around or over them. They represent remoteness and difficulty. They are barriers to be overcome.
Ideal things for adventuring characters? Perhaps. Adventures can be found in cities, though, not just on the side of a mountain.
The other quirk of my writing is I like nature to rain on my characters. This is a bit easier to answer. I found that environmental effects on outdoor adventure stories can be a bit muted. In a fantasy setting, rain is both more and less inconvenient than today. Playing it up adds a nice realistic feel even as they might be throwing magic at each other.
Rain and mountains. An adventure is supposed to have hardships. A mountain journey can be difficult. Adding rain will make it more difficult. This is good for character development because you now have good excuses for short tempers. And that makes for better writing.