Because saying "yes" can be self-sabotaging behavior.
At the beginning of this year when I was settling on some good New Year's Resolutions, I had decided that this year would be my "Year of Yes." I would say "yes" more often to doing things even if I didn't feel like it and to things when people asked me for help.
Caution: When you say yes to everything, people start to unknowingly take advantage of your kindness.
I say "unknowingly" because many times I think people are more likely to ask people they know are likely to say "yes," not to take advantage of them, but to solve a problem more quickly, whether it be filling a shift or going out to dinner and spending money you don't have. The problem with "yes" is that you may take on too much to handle. That stress turns into resentment and you start to channel anger and a whole cocktail of emotions towards friends that do not deserve the hate.
In my case, the stress would turn into me procrastinating on the work at hand or procrastinating on more important tasks that I would even sometimes turn in late because I couldn't handle the pressure and gave up. I couldn't go out to parties or music festivals on the weekends because I would leave homework until the last minute and it caused my roommate and me to have more of a sub-par time at school than we should have experienced.
What things did I say "yes" to that I wish I had said "no"?
It's not ideal, but a lot of people have to work while in college. I've missed out on a lot of events because of work and because I said "yes, I can handle extra shifts. I need money." I missed out on free movie previews, events on Mill Ave before football games, and much more. After working so many days each week and going to class, the last thing I wanted to do was homework. It put me in the worst mood by March and I was counting down the days until summer. I had lost my love of learning and motivation for my favorite classes, when last semester before I had a job, I'd spend days in the math tutoring center. Working a job should always come second to school, though many times it has been my primary priority and probably yours too.
Those days of eating out and spending too much money on food that I shouldn't have was when I should have said "no" too. Buying food out of laziness is not fun. You eat it, the satisfaction dissipates, and now what? You lost $15 to a subpar salad with chicken. That doesn't even include the tip. If you're working for an hourly wage, was that meager salad worth two hours at your job? Definitely not.
These may not be your personal struggles, but you get the point.
We all need to be saying "no" more often for our mental health, our wallets, and to live the life we want to live.
After all, you are living your life right now. Do more of what you want to do and start saying "no."