We now have a thread for it's too hot and another one for it's too cold.
The thing is, if it's too cold, then you can put on an extra layer, keep a coat in your bag/rucksack, carry some soup in a flask, and generally just check the weather forecasts and be prepared.
If it's too hot however, and you've checked the forecasts, you're already down to 1 layer, nowhere near air conditioning or the shade, then you're screwed.
It's easier to get warm if it's too cold than it is to cool down when it's too hot.