To reach the goals you've set for yourself. When you limit the amount of tasks you commit to, you are more likely to actually complete them, and you’ll do so with more efficiency.
To rid yourself of undo stress. If you take on too many tasks, you can cause yourself to go into overload, and then end up doing nothing at all. Rather than doing 10 things with a sour burnt-out face, why not just do 3 things really, really well.
Because you just can’t do it all. Although you may find a way to rescue every person and help with various tasks, you just cannot lend yourself to any and everything. It may seem like taking on an additional task won’t be too much trouble, but there are only so many hours in one day, and you have to find balance.
To have more energy to do the things you want to do. Be selective in what you say yes to. Choose to say yes only in areas where you know you will be an asset rather than a liability. By doing so, you will have time and energy left over for you and your loved ones.

The “Perhaps next time” NO. If you really do want to help with a task, but are just simply unable to at the present time, then politely decline, and let the other party know when you might next be available to assist them.
The “On your own terms” NO. This is where you determine just how much or how little you’ll commit to any given task. You can say “I can help until noon but then I’ll have to leave afterwards.” Remember, you are trying not to overload yourself with extra responsibilities, so set your own terms to what you agree to do, or just don’t do it at all.
The “I’ll get back to you” NO. Don’t respond immediately to requests for your assistance. Take a step back, and give yourself some time to respond. Tell the other party you’ll think about it and get back to them. You’ll save yourself any future heartache.
The “I-understand-your-dilemma-but-no” NO. Sometimes, someone else’s story may pull on your heartstrings so much that you give in, and take another task on. It’s ok to sympathize, but remember that it’s your time and it’s important that you manage it based on what you can actually commit to doing.
The “cut to the chase” NO. In some cases there is just nothing else you can do but to stand firm and plain ol’ say no. A gentle but firm “No, I’m sorry I’m not going to be able to” is sufficient.