Why are streaks so effective?
The streak (consecutive day series) exploits loss aversion. Psychologically, losing a 30-day streak hurts more than not having one. That's why Duolingo, Snapchat and Expert Dactylo use this mechanic: it transforms a fragile habit into emotional commitment. The longer your streak, the more motivated you are not to break it.
How do XP points maintain motivation?
XP (experience points) create a sense of continuous progression. Even a bad session earns some XP. The brain perceives each gain as a step toward the next level, which maintains the desire to continue. The key: the progression curve must be well-calibrated — fast enough to gratify, slow enough for each level to have value.
Do badges really change behavior?
Yes. Badges act as "identity markers". Unlocking the "7 consecutive days" badge makes you think "I'm someone who's consistent". The "60 WPM" badge identifies you as "fast". These micro-identities reinforce the associated behavior. It's subtle but powerful.