why is it that some people like deceiving themselves?

I wouldn't say that putting a brave face on things was deceiving themselves, I would say that it was because some people don't like to burden others with their problems. Maybe they have been hurt in the past by confiding in people who threw it back in their face later on? It is hard to know who to trust.

it makes them more calm or be optimistic.

it helps them believe they are still in control and in some cases it works

i think there's a "self-efficacy theory"/ "self-fulfilling prophecy" you can look up

or the person just doesn't want to face reality= fear

or just taking a breather before jumping into a point of no return

downplaying a situation could also mean the person is breaking down the facts so he/she could think more logically (less emotional) and decide better

Because sometimes it is too painful to admit to ourselves what we really feel or what is really going on for us. It pften seems easier somehow to bury our heads in the sand and pretend everything is OK.

all humans have that capacity and many often do not ( most times) even know they are doing so. it is a survival function. by refusing to see or react to some things we can better deal with them or ignore them .thus enabling life to move on without too much emotional or other upheaval or need to do anything.

We all deceive ourselves. Including you. You're in denial.