The best part of teams is that they are made up of human beings...the worst part of teams is that they are made up of human beings!
The motivation of a team will vary depending on the emotional state of each person, and often it is outside influences that affect people's mind fitness.
If you see someone upset or they come to you, give them some time. First, get away from your desk, show them that you value how they are feeling. Take them for a coffee, get them away from the telephone, e-mails, other people. Maybe even take them off site - I would go for a walk around the block, or go to a local coffee shop.
People say that they haven't got the time, well you haven't got the time for that person not to be productive, not to be in the right mindset, affecting the rest of the team. Imagine if the problem means that this person will leave - imagine the time you would have to invest in replacing that talent??
Your role is to lead people, and valuing their feelings pays huge dividends in the long term through commitment and loyalty.