Is there anything that motivates you less to get up and train than having a night of little to no sleep as the temperature has resolved to not dip below 25 overnight, there's no breeze and you know that it's forecast to hit the mid to high 30s or worse again that day? You're already sweaty and tired, so why should you be bothered to get up and and add dehydrated and even sweatier and more tired to that list?
It's just not an enjoyable experience training when it's really hot. Sure, sometimes the inspiration will just hit you and you'll really feel like going for a run, or doing some other workout - but for the most part, especially when it gets really hot, your body and mind will both be telling you to ignore it all, go somewhere nice and airconditioned and just relax. I know that's how it is for me.
Yesterday I ran headlong into the other issue associated with training in hot weather - dehydration. Despite downing nearly 2L of water over the course of a one and a half hour session I'd dehydrated myself badly enough that my skin had lost much of it's elasticity and I split my middle finger open along one side while carrying my bag back from training (the simple act of fingers rubbing together did this). The severe dehydration also made itself known through the other side-effects of reduced cognitive function and motor skills as well as very dry eyes and other symptoms.
After an experience along those lines it's easy to say: "bugger that", and simply put off any training until it's not as hot, but the fact of the matter is that once you find an excuse to skip one session, you'll find it easier and easier to come up with more along the track.
I know that I'll often think about not training or search for reasons not to go, and it takes a conscious effort to over-ride such thoughts and recover the motivation. For everyone reason that you can think of not to go, try and think of a reason why you should. Most of the time you'll run out of reasons not to go far before you run out of the reasons to go.
And if you do crack and skip a session of training for whatever reason - make up for it somehow. Even if you do something that isn't as intense or as long, make sure that you still do something. I've found that inertia is my biggest friend on the training path - as long as you can keep on doing something, it makes it easier to keep on doing the right thing.