Why "to" is dropped after "is"?

Question

Why is 'to' sometimes dropped in these kinds of sentences?

What you want to do is make a plan

There's no 'to' between 'is' and 'make'.

The next thing you want to do is to stay in the present moment.

There is 'to' between 'is' and 'stay'.

Similar example

The first thing you need to do is to trim vegetables.

The first thing you need to do is trim vegetables

Answer 1

I believe this is a case of the "zero infinitive".

Essentially, either usage is correct, though there may be a preference for the zero infinitive case ("The first thing you need to do is trim vegetables"). Using the to-infinitive case ("The first thing you need to do is to trim the vegetables") tends to sound unnecessarily formal.