We often leap from experience to label: he doesn't look me in the eye -- he must be shy. She took the time to help me; she's so nice. While it seems nice to say, "you're so nice", the listener can't help but try to match our characterization with her own view of herself.
Share your experience of the other person instead. Give the person a chance to understand how you experienced his actions. Demonstrate what you value about the other person rather than what kind of person you want him to be.
Example: "I appreciate your taking the time to help me, Priya. I know I was a tough student, but your walking me through the steps gave me the confidence to use the software on my own."
Share with the other person what they do that matters to you, rather than define them.
Adapted from The Seven Languages for Transformation: how the way we talk can change the way we work, by Lisa Lahey and Robert Kegan. (c) 2001 Jossey-Bass.