It is rude to waste someone's time.
Sally is at an ice cream shop with lots of different flavors.
Sally wants some ice cream.
Sally only picks between a few flavors, so she doesn't hold up the line.
Sally insists on tasting every ice cream in the store, causing a backup of the line.
Sally gets a flavor she likes, and the store is able to serve the customers in a timely fashion.
The customers get mad at the wait, and the server tells Sally she needs to choose now or leave.