Buddhism is a practical philosophy – it's meant to be tried and tested in the "real" world. It's designed to help us understand where we get in our own way to happiness. To newbies, it sometimes sounds pretty depressing when we talk about the reality of suffering and the path to liberation from suffering. To me, I actually have a harder time with the teachings about joy.
One set of teachings are on the Brahmaviharas (the four heavenly abodes or the four limitless qualities), which include compassion, loving-kindness, unselfish joy, and equanimity. These teachings are designed partly to help us break our bad and selfish habits. Specifically, when we encounter situations with other people, we tend to make them small – to only see how they affect "me." So if someone is suffering, we may have a hard time feeling compassionate because we become judgmental or superior. If someone gets a promotion, we may have a hard time feeling joy because we become envious or cynical. We tend to think that whatever we encounter is "happening to us."
I once was at a job function and was asked by a colleague what was new. I had just been invited by a publisher to write a book. The colleague said to my face, "Well, the only reason they asked you was because they couldn't get someone good."
This example shows what we do so easily – when we see something good happen to someone else, we immediately turn it around as if it was about us. We often treat happiness as if it is a limited commodity and a zero-sum game. If you have it, then there's less for me. This is silly. If I tell a joke and 10 people laugh at it, the 10th person doesn't have less because there were 9 others laughing first. In fact, more people being happy increases the laughter for everyone. In the workplace, however, it's often very difficult to feel happy at others' good fortune because we come with such a limited mindset.
The good news is that we can use those habitual reactions to clue us in to when we could try something else. In the face of something that could be joyful, we might recognize ourselves judging, comparing, belittling, criticizing, or resenting. Criticism is often an expression of jealousy. Jealousy is often an expression of a feeling of insufficiency. We try not to look at that honestly, and instead cover our neediness with criticism, sarcasm, and snarkiness. When we find ourselves making sarcastic comments, it's a clue that something else is going on. Can we see beneath our outward expression to find what it is? Can we go past our habitual self-protective reflexes to try to really feel glad (or compassionate) to our co-workers?
In our stresses at work, we like to focus on all the things and people "out there" who are doing things to us, but our habitual reactions keep the cycle of unpleasantness going. Being happy for others' successes will leave you feeling less resentful, and will make you better company for your company.