I enjoy riding the bike, and feel like I'm missing something when I can't do it. I look forward to riding, it makes me happy, and it melts my stress away as an added bonus. I don't need to find the motivation to get on the bike; if anything, I sometimes have problems getting out of other obligations. But, frankly, I'm greedy about this, so if my GF wants to watch a movie or hang out with friends, I tend to meet her after we're both done doing our own things.
If you don't feel this way about cycling, perhaps there's something else that will get you outdoors, enjoying the fresh air, and pumping blood...?