Wow. I understand why the app succeeded. 1) It has a simple and beautiful UI. 2) It uses a Skinner Box system effectively. Do task, get reward of seeing it completed on check box, miss once and you don't get as much of a reward (assuming I'm interpreting the app correctly). 3) People like to think that they are doing more than they are actually doing. For example, people would like to think that if they work on something every day for a year it has to get done, but if they are doing things so small (for example, 1 line of code a day) it won't get done. However, from a perspective of Streaks it "looks" like more work is getting done than what actually may be getting done