It's really easy to avoid exercise.
When the weather is off-putting, be it too cold or too hot, raining or snowing, it's far too tempting to take the easy option, like staying in bed, or using the car rather than the bike.
Then there's money. One of the reasons I started adding a bike ride, run or walk to my commute was to save money. When I was exclusively working for myself and commuting to and from my own office it made sense, but now that I don't, and my employers pay my travel expense, it's less of an incentive. Plus the way that the train lines I use work mean that getting off a stop earlier doesn't make any difference (financially-speaking).
Another thing that's affected my dedication over the years is where I exercise. I don't like running or cycling on roads. I much prefer trails for running and cycle paths for cycling. As that's been my clear preference for years, it makes sense that I don't really relish a stroll or ride along roads with traffic whizzing past me.
There's another underlying cause: a lack of discipline. You might feel super-motivated, but without discipline you're more likely to throw in the towel at the first obstacle.
Don't give up
Giving up has made me feel bad every time, physically and mentally. As someone who has suffered with depression, I can vouch for the positive effects of exercise, and the negative effects of being idle. This isn't me just spouting personal opinions, it's backed by science. Harvard Medical School (among many others) reports that:
"In people who are depressed, neuroscientists have noticed that the hippocampus in the brain—the region that helps regulate mood—is smaller. Exercise supports nerve cell growth in the hippocampus, improving nerve cell connections, which helps relieve depression."
This is where discipline trumps motivation. You can't go out for a walk once or twice and, upon not suddenly feeling better, make up your mind that exercise isn't helping. It will, you just need to give it time.
As the article quoted above notes, "this is a long-term treatment, not a onetime fix."
So keep going. It's well worth it.