Why is bullet so addicting?

Because each game can be decided at any moment of time.
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- You could lose on time by playing too slowly (even 1 second per move is too slow).
- You could lose in terms of position or you could hang pieces if you play quicker than 1 move.
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You always fear the loss of time, leading to an increased desperation of moving pieces. The fast-paced nature of bullet is why it can get addictive for a proportion of players.