For someone as naturally lazy as myself, if every time I perform any kind of mitzwah it is a life-long commitment, then my yetzer hara has a very good chance of getting the better of me: "It might seem like a small mitzwah, but are you really ready to do it all the time, it's an added responsibility, it's another bother, are you really sure you want to get yourself into this?"
If, on the other hand, I tell myself: "this is an isolated act, I'm going to do the right thing this time, but I don't think I'll be able to do the right thing the next time," then my yetzer hara has a lot less to work with. Why shouldn't I do the right thing right now? There's no excruciating long-term commitment involved, in another minute it will be all over.
If I approach each mitzwah of the day this way, it's hard for the yetzer hara to get an in. Of course, the yetzer is more wily than I, so I don't think this is a long-term strategy, but it helps me when my energy levels are low, and I feel a strong desire to be lazy.