Fogg states that all humans are motivated to seek pleasure and avoid pain; to seek hope and avoid fear; and finally, to seek social acceptance and avoid rejection

Users who continually find value in a product are more likely to tell their friends about it.

When designers intentionally trick users into inviting friends or blasting a message to their social networks, they may see some initial growth, but it comes at the expense of users' goodwill and trust. When people discover they've been duped, they vent their frustration and stop using the product.

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.

As fleeting commitments, diets often fail. Thinking of dietary choices as part of who you are...can give them real staying power

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

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.