Being agreeable can be a great trait that helps to establish working relationships. In fact, being agreeable can be so advantageous that you start to fill up your calendar with interesting projects and ambitions that wouldn't be possible otherwise. This process is essential for development in my experience, especially as a student. But for many of us, this track record can cause some problems down the line.
As your schedule fills up, you can start to feel the pressure associated with saying yes to another project. When this starts to happen, I believe it's still advantageous to commit in most cases. You can't grow and learn to take on more responsibility without getting outside of your comfort zone. This article isn't meant to promote laziness, lack of responsibility, or inhibit self-development. Rather, you have to know when the cons of taking on more work are exceeding the pros.
When deliberating whether to say yes or no, you can assess a hierarchy of considerations based on what matters most to you. I've split this list into "selfish" and "selfless" considerations, by which I don't mean to assign any connotation. Some selfish examples include, does this benefit me, am I interested in this, will this diversify my experiences, will this lead to a meaningful relationship, do I value this connection, and will this take away time from something else that I care about?
On the other hand, you should always assess work as it pertains to others. For instance, will this meaningfully help someone, does this person deserve it, and is this person putting in the effort? You should be willing to make reasonable sacrifices if they will help others. For instance, what I've listed are some things you can consider when taking on a mentor role. I'm personally very lucky to have a long list of mentors and role models, and every one of them sacrifices time to help me. I try to do the same for current students whose shoes I so recently filled. Not to mention, many selfless acts can be viewed through a selfish lens. By helping others, you're likely helping yourself at the same time. It feels good to volunteer and watch someone develop over time. Being a mentor is an important part of self-development and will help unlock skills you didn't know were important. Also, by helping someone you enhance their ability to make a positive impact on the world, which again, helps you.
All of these selfless considerations apply outside of the workplace and to your family as well. You should always try to help your family, friends, and community if possible. Many evolutionary psychologists will tell you that there is immense value in such endeavors. Of course, here too lies a hierarchy. For instance, you can't help others if you can't help yourself. This is why you're supposed to put on your own oxygen mask before your child's in an airplane emergency. My personal hierarchy goes something like this: self, family, friends, community. I take self-development and personal health (mental and physical) very seriously and therefore commit a lot of time to them. This ensures that I'll always be emotionally and physically capable of being there for my wife and family should they need me. Their happiness and success are extremely important to me and there's very little I wouldn't do if someone in my family asked for help. Outside of family, having close and meaningful friendships can lead to genuine enjoyment and meaning in life. However, all friendships are bi-directional and you have to make sure both parties are contributing to them. Then there's the community. Once you have all your ducks in a row and you're reliable to yourself, family, and friends, you can start to help a larger group of people. Many of the people I respect most are those who have done the self-work necessary to spend tremendous time and resources helping their community.
Between professional and personal commitments, you'll eventually get to a point of diminishing returns where saying yes can cause more harm than good. That harm can come from stress, opportunity cost, or guilt as you'll now have less time to spend with friends and family. If the cost of saying yes exceeds the benefits, then you should feel comfortable declining.
When it comes to saying no, you should do so in a way that is not offensive, but final. You have to relay the message that you wish you could prioritize their ask, but your current commitments have left you without additional capacity. If you're lucky, you'll find yourself in a work environment that is cognizant of personal capacity. The same can be said for your social environment. You shouldn't feel guilty about saying no to something, specifically when you're doing it for the right reasons. One of the best ways of ensuring you don't offend someone by declining is to have a robust record of acceptance. As I've said, when someone knows your history of repeatedly committing, it builds a strong working relationship. That person knows that you aren't shirking off work or being lazy because it would conflict with their vision of your character. The best way to say no is with a history of saying yes.