“Boast not thyself of to morrow; for thou knowest not what a day may bring forth.”
Do not become a bragger, that is what we are talking about here. No one likes to listen to someone carry on about themselves all day long.
Further, you do not know what tomorrow may bring you. Today things may be well, tomorrow… It is promised to no one.
We also need to read verse 2 to ensure we are humble people.
2 “Let another man praise thee, and not thine own mouth; a stranger, and not thine own lips.”
If you are truly worthy of some sort of praise, other people will do the talking for you which provides a witness to whatever your works happen to be. Maybe you are a hard worker, maybe you run a good business. Through those good works, others see that and they want to share with other people there good experiences.
This goes for everything in life, including sharing our Father’s Word with others.