Saying yes is easy but if this leads to more work than you can manage no-one wins. You may not do the best job and it can lead to resentment and burnout.
Before answering any request give yourself time to consider it.
What is the commitment?
How strongly do you feel about the project?
Is the request in your job description?
Are you the best person for this job or just the one most likely to say yes?
DO NOT say yes out of a sense of guilt or obligation. Guilt is a destructive emotion that will generate bad feeling.
It is ok to say no. If you don’t have the time or inclination for the job you are not the best person for it and it is better to say this.
When you have said no do not give a reason why not as this invites discussion. Just say No!