On occasion I omit commas. On occasion, I use them. (The more you know about English, the less you're likely to think there are unbreakable "rules" for a lot of these things.)

Failure is part of success, an integral part. Everybody gets knocked down. Knowing it will happen and what you must do when it does is the first step back.

Your enthusiasm becomes their enthusiasm; your lukewarm presentation becomes their lukewarm interest in what you're offering.

I have a sense of humor. I usually come off as very serious, but I definitely have a dry sense of humor.

Many people erroneously think they have only one chance to succeed, and if they miss that chance, they are doomed to failure. In fact, most people have several opportunities to succeed.

Your attitude will unlock the mystery of success you seek!

The dash helps to indicate that the two thoughts are intimately related, and it's less stodgy than a semicolon, which would have performed the same function (and who talks in semicolons?).