Happy to Be Guarding against Failure
Other testers are happy upon finding problems because they know that by finding problems early, they are able to help the project stay in touch with reality. The sooner they are found, the sooner they can get fixed. True, the problems might've been found sooner, but these testers know that their efforts have allowed them to be found sooner than otherwise.
They report problems in a neutral manner, making clear the impact they'll have on users without resorting to a blaming attitude. This helps the project team determine how best to handle the problems. They also know that the reason they've been hired is to find these bugs. The plans may have been optimistic, but the testers were hired and put on the project specifically to guard against the dangers of exaggerated optimism.
They know that by effectively finding and reporting problems, they earn the respect of programmers and managers. They feel good about the problems they find, because it means they're being effective in helping the project.
These testers have an attitude that allows them to feel good about their work and also a part of the team. It's not an easy attitude to maintain consistently, but nonetheless key to their effectiveness.
Feeling Your Feelings
So, how should you feel when you find a bug? The truth is that you need to acknowledge whatever feelings you have. If you are annoyed or dismayed, then you get to feel that way. You may, however, need to be diplomatic when sharing your feelings with others.
Ultimately your effectiveness and satisfaction will be best when you can feel good about good work and when you don't have to hide your feelings from your colleagues.
This requires a mature attitude regarding your role on the team. But it also requires a mature organization that understands the bind in which testers are often caught. No tester is going to be able to feel good about working for an organization that punishes testers for finding too many bugs.
One great way to communicate a mature attitude is to respond to bug reports with "Thanks for the bug report."
This may start as forced politeness, but eventually everyone gets the idea that reported bugs are better than unreported bugs. Testers can say this in reply to bug reports they get from other team members (even if they are already in the bug database). Managers can say this in reply to reports they get from testers (even if they don't agree that it's a bug). Programmers can say this too (even if they've already fixed the bug in the most recent build).
What other practices have you seen used to help reinforce positive attitudes regarding the tester's role on the team?