There are 2 times you say yes to a habit.
The first is when you initially put it into practice and try it out - do you even enjoy it? is it suitable to the way you live? will it actually give you the benifits you need?
Then there is the second - the dip. When you've kept it up for a while but it begins to grind on you. Maybe over the long term it just isn't viable? The initial excitement is gone and now it's just very boring, it takes too much time...
You forget your purpose.
And that stops you from continuing.
You forget that purpose is what you feel from doing it, that it is gained, not a prerequisite.