Babies take off their diapers for a variety of reasons.
The baby that takes off their diaper could be doing so because they are wanting attention and like your reaction that you give them when you see them take off their diaper.
The baby may also take off their diaper because they don't want to wear diapers anymore and are ready to potty train.
Or the baby could be uncomfortable wearing a wet or poop diaper so they take it off because they hate the feeling.
If it's because the baby is not liking wet or poop diapers be sure to change the baby more often especially if they are not of the age to potty train or they are not ready to potty train.
Some babies are ready to stop wearing diapers and are ready to potty train as early as 18 months old so if they are at least 18 months old you could try potty training them.
But it doesn't always mean the toddler or baby is ready to potty train just because they remove their diapers.
If your toddler is removing their diaper and they are not ready for the toilet and they need to wear diapers make sure they have a clean diaper often enough and use some little keeper sleepers or have them in clothing that is harder for them to get to their diaper.
If they can't get to their diaper they cannot remove their diaper.