I tried to be one of those reasons for about two years. I fought my husband every morning he slipped out of our warm bed and put on a pair of the shoes he loves so much more than me, though he won't admit it. I could not understand the pull that running had over, what seems to so many, a more attractive choice.
Then I became a runner and I admit there might still be days when I am still in the warm house looking at the cat who is cocking his head just slightly in the judgemental way that says "hey do you know it is 22 degrees out there and 70 degrees in here. Plus we get canned food this morning." It is also fair to tell you, this runner at least, only thinks that while still in the house. Once I am out the door and my running partner, my music, or that cool outfit I bought just for this weather beckons I forget all about that smug cat and start doing what I do.
All of that said does not really answer why. I think the unique thing about runners is the answer to that is "it all depends on the runner." I don't know what drives my husband to run everyday and be closing in on a two year running streak, especially when in that two years I have seen him in tremendous pain from injury and still get out to do his mile or even more impressive run a race. I guess the question is why do I run?
Because no one thought I could, especially me. I feel incredibly strong when I run and I feel like I can accomplish things I did not think I could, like say: running.
I would say even if your thing isn't running you have to have a thing. Something that makes you ignore the cat and get out the door and feel like you accomplished what you and everyone else thought you could not do. Find it and be like my husband; fight the pain and inertia and just get out and do it. I am trying to be more like that and it is not as hard as my cat thought it would be.