T cell undergoes maturation in the thymus and one of the tests it must pass to mature into a single positive mature naive T cell is positive selection. In positive selection your body is making sure that the T cell that it produces is reactive to your own MHC. If it cannot bind to your own MHC, the T cell is useless and it will just die by neglect in your thymus. In positive selection the T cell is "tickled" with thymic endothelial cells that express your own MHC class 1 and MHC class 2. If there is an affinity of the T cell to bind to your MHCs it will continue to the next step in maturation which is negative selection. If the T cell binds way too strongly to your MHC it will also die. There is a specific range that it must bind to your own MHC for it to continue.