Think about when you’re asked by someone to build an exception. To move their request to the top of the stack. To bend the rules. It’s totally up to you what you say “yes” to, but for most of us, it’s a default reaction. It’s human nature to want to please others, and saying yes to others’ requests is an easy way to satisfy that need.
Just remember when you say yes to one project, you’re saying no to others. Even if you don’t recognize that transaction now, it’s still taking place.
The easy solution to this is to delay the answer a bit. Not long, maybe only a few hours, or a day or two. Enough to look at the request alone, without the social pressure, and separate the desire to please someone with the hard facts of the request at hand.