Others have mentioned this, but for me, the humidity has a lot to do with it. The hottest temps I've ridden in (between 60-80 hilly miles/day on a tour) were consecutive days of 105 degrees F. But, the air was dry, and I loved it, when others were dropping like flies.

I also love cold, dry weather. As long as humidity is low, I'm fine with very cold temps.

But, in our 80+ humidity, even 60 feels freezing to me on the bike.