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Understand when to say no. Learn why it's hard to say no. Accept your power and importance. Accept that others are the same. Understand that “no” isn't cruel.
Two-year-olds are famous for saying “no” like it's going out of style, because they've only recently learned that such a thing is possible, and the new chances for independence it provides are fun and exciting. Two-year-olds are also known for being selfish and thoughtless. However, they're on to something: it's okay to say no. What separates adult usage of the word is that we can learn when it's appropriate and when it isn't.  Saying no when you just don't feel like doing something is okay, as long as the thing you're being asked to do won't reflect on your job or school performance. There's nothing wrong with wanting time for yourself instead. Saying no because you don't have the time to meet a commitment is okay. Others often don't realize how difficult making such a commitment would be, given your schedule; some people do know, and are only asking just in case, even though they know you'll probably decline. Saying no to a situation that makes you uncomfortable is perfectly okay. You never have to step outside your personal comfort zone to accommodate the wishes of anyone else (except, perhaps as an active-duty soldier following orders). Saying no when you're asked to buy something is okay. There are many different specific reasons a person might find it hard to say no to others, but the common thread that ties them all together is worry – worry about what the outcome will be if you say no. It's normal to worry about decisions that you make, but it's important to understand two things: first, worrying won't change what happens after you've made your choice; second, worries should never stop you from acting in your own best interests to begin with. No matter what your reason for being scared to say no, it stems from your worries about what will happen when you do. Will people still like you? Will you miss an important opportunity? Will you seem lazy, uncaring, or incompetent? Acknowledge that you don't say no because you worry, then accept the fact that worrying never helps anything, regardless of the outcome. Like a piece in a jigsaw puzzle, you're a vital part of the landscape around you – it wouldn't be complete without you in it. This applies no matter whether you're always out with friends, or you sit at home and hide all day. The fact is, no matter who you are, your presence in the social landscape is valid. Furthermore, the decisions you make have an effect on that landscape. This means you owe it to yourself and those around you to make honest decisions, for the betterment of everyone – whether they realize you're helping or not. Worrying about what will happen when you say no is symptomatic of a larger problem: worrying about the power you wield over those around you. Acknowledge that you wield that power no matter what you do or say. Though people vary in terms of personality, opinions, and attitudes, one thing they all have in common is a presence in the social landscape around them, same as you. It's an immutable fact of living in human society. Therefore, controlling and channeling your own presence in such a way as to help you be happy is really the only sensible choice there is. It's not as though you have some great and terrible reserve of influence nobody else has: if you say no, you're only exercising the same power everyone around you also has. How they react to your decision is their business, not yours. You have every right to set boundaries for yourself. After all, your friends do, and people still like them. In fact, being assertive or even aggressive about what you want won't make you hated or despised. The only thing that will make that happen is openly treating those around you as though they're inferior. Saying “no” isn't an expression of superiority; it's an expression of mutual respect. By itself, saying no isn't rude, mean, or uncaring. We attach those qualities to it when we speak in a rude, mean, or uncaring way while telling someone no. There's no reason you can't firmly decline and still be pleasant and polite; therefore, there's no reason to fear making a poor impression because you say no, as long as you're mindful of how you say it. In other words, once you understand that it really is okay to say no, the rest is just learning how to say it politely.