I'm fluent in the language of "yes".
I use it every day, multiple times, in its various forms: "yeah", "sure", "of course", "you bet", "absolutely", and my favorite, "anytime". For the most part, I really love it. Accepting various propositions will usually open wonderful opportunities to meet new people and do new things. Also, agreeing to such allows me to gain favor with the person asking. Using these affirmatives can be very beneficial to relationships in both the personal and career realm because people like to feel connected. People like to work together and people like to ask others to join them in their cause, in their activities, and in their pursuits. I've learned, through life's training, that: people really like to hear the word "yes".
The trouble with saying "yes" to working toward everyone else's goal, is that we can lose sight of our own.
Everyone has the same 24 hours in each day to accomplish what their heart and mind tell them needs to be done. If we are allocating the best of our hours to other places, it leaves minutes left to devote to our own goals and our purpose in life. If we spread ourselves too thinly, we fail to make a true impact in any area. If we exhaust the majority of our time to other places, we can wind up underutilized in the pursuit of the goal which we have been called to reach for. Which is why we need to learn to say "yes" when we can, and "no" when we truly cannot.
So, how do you know when to say "yes" and when to say "no"? I believe the answer to this is as simple as knowing where you are going in life.
Identify your mission. Be firm in your purpose.
Every organization identifies a mission statement and vision which helps articulate it's overall purpose and passion to the public. We as individuals should have no less. It's not enough to only have passion, we also need direction and a clear framework of how we will carry out those drives. Ask yourself: "what am I here to do and why?" Then live by it. When we firmly express and live by our purpose, others will know where our commitment lies.
Establish clear and specific goals. Detail your objectives.
At dinner the other night, a friend asked me: "Where do you want to be in 10 years?". At first, I somewhat scoffed at the question with a cynical remark about how I can barely keep track of where I'll be in 10 days. However, as I thought more about it, I realized how important it is to set goals, both long-term and short-term. If you don't know where you are going, you can end up anywhere. That unplanned uncertainty might be fun for road trips, but do you really want fate to determine the direction of your entire life?
Finally, when you receive a proposition from someone:
Choose to say "yes" when it aligns with your vision and integrity.
We've established that you can't say "yes" to everything you're asked for but, let's be honest, you can't say "no" to everything either. There are some very good advantages to saying "yes" if you are carefully selective with the opportunities which arise. Be sure to first compare the proposal's mission with your own to see they are in line with each other. Is this going to add to your personal integrity or take away from it? Do you have adequate time to appropriate to the cause without shorting yourself? Will this cause you joy or stress? In short, know when saying "yes" to something really means you say "no" to yourself.