I too, am the opposite. I really prefer it to be below freezing for running. Summer heat and humidity where I am are huge barriers to overcome, runningly.

My initial motivations for taking up running in my 20s were impatience and fitness. I worked a long walk from my (urban) home at the time and got bored with how long it took to walk. I was also pretty unfit, and running in transit seemed like the simplest way to exercise, so I started making increasing monthly running goals.

Since then? I mean, fitness is theoretically the prime motivation. But practically, inertia is probably more salient - years of consecutive perfect success at monthly goals are a pretty darn good motivator all by themselves.