Many people don't realize it, but a big part of true leadership is the ability and willingness to identify, accept and share yourself as a whole person - not just someone who works perfectly in an office.
When we think of great leaders, we often think of their decisiveness. I define leaders as people who make things happen. What that boils down to is, to lead you decide to take action.
If people do say no, they usually do it in ineffective ways that come with an excuse. For example, they might say, "I'd like to help but I'm really busy". The problem with this approach is it gives the other person an opportunity to continue to ask. They feel they have an opening. "Since you're busy this week how about next week?"
My clients often fear that if they show their true selves at work and reveal to their peers the fun sides of their personalities, they won't be taken seriously. If your professionalism makes you feel stymied, then you've probably taken it a bit too far.