Part of what I learned about vulnerability in the brene brown’s audiocourse “The power of vulnerability” is to let people know your thoughts.
Not all of them, but it’s necessary to let them know what influence your decision.

Today I talked to my boss about a side project. I talked to him because, since I’m working part time (and it was my choice) I didn’t want him to think that I was trying to get extra work.

It is very important to be clear upfront, to show the cards before anyone can make a comment.
This is because, as it often happens, we feel betrayed whenever someone didn’t tell us something we would’ve liked to know.
And it’s a bit hard to predict which are the infos that people want, so better include some extra info everytime, this way you’re at least sure that you tried to minimize the issue.