Habit-forming products alleviate users' pain by relieving a pronounced itch.

Products with higher user engagement have the potential to grow faster than their rivals.

To initiate (user) action, doing must be easier than thinking.

If users are not doing what the designer intended (when users are investing time, effort, etc in your product), the designer may be asking them to do too much.

For an infrequent action to become a habit, the user must perceive a high degree of utility, either from gaining pleasure or avoiding pain.

Companies leverage two basic pulleys of human behavior to increase the likelihood of an action occuring: the ease of performing an action and the psychological motivation to do it.

You'll often find that people's declared preferences - what they say they want - are far different from their revealed preferences - what they actually do.