When the other disciples tell Thomas the Twin, "We have seen the Lord!" it must have been an emotional moment. For most of the group it was a transforming experience, their despair morphing immediately into joy, and they were high on the power of hope. Thomas, though, must have had a range of emotions. He was no doubt shocked at this surprising news, and lost his bearings for a few moments. Likely he felt a glimmer of his companions' excitement fighting with his cautious nature and protected himself from being hurt further. There was also a twinge of feeling left out, which often provokes a bit of defensiveness.
Reading his story again I notice two things. His resistance -- "I won't believe it unless I see him, and touch his wounds" -- isn't really directed at Jesus, but at the disciples' report. It's not surprising considering what he has gone through. First, John 20:9 makes clear that the disciples weren't really expecting the resurrection. Mary Magdalene was so distraught at finding Jesus' body gone that she conversed with the risen Lord but didn't know him until he named her. Her experience led her to tell the other disciples -- it isn't clear if Thomas was there -- so they were prepared, in a way, for Jesus to appear to them while cowered behind locked doors.
Thomas is like many people today. He hears about the others' experience of Jesus, but he demands experience of his own. Or as has been observed about "postmodern" people's reaction to the Church, they want to experience Jesus' love and power in the life of the Christians they see, not just hear information about Jesus that they're told they need to know. Thomas isn't overly stubborn -- clearly he shouldn't have known this was going to happen -- but he is looking for his own experience of Jesus.
The Messiah, for his part, knows this, too. And when he next appears to the disciples, he directly addresses Thomas' specific questions, "proving" that Jesus has been with Thomas even when Thomas wasn't aware of it, and creating a uniquely personal experience of Jesus for him.
It's easy to be like Thomas. I find myself frequently thinking things that sound like him: "Lord, I think you want me to go this way but I won't really know unless you show me." When I know but I don't know, I'm really asking for the knowledge to come out of experience, out of relationship, rather than just out of my head. Is that really different than what most people on a spiritual quest are asking?
Those of us who, unlike Thomas, believe without physically seeing Jesus are blessed. But if Christians are sent out just as the Father sent Jesus, then the people we run into in the world should experience through us God's care and compassion for the world, just as people experienced from Jesus. Otherwise, why should they believe our story?