The primary reason temperatures of the order of 107K are required to initiate a controlled deuterium–tritium fusion reaction is that
(a) At about 107K the plasma is completely ionised so that, with electrons stripped away, the bare nuclei can fuse.
(b) At about 107K the neutron absorption cross-section of deuterium and tritium becomes large, allowing the reaction to proceed.
(c) The temperature gives deuterium and tritium nuclei enough kinetic energy to overcome their mutual Coulomb repulsion and get within range of the strong nuclear force.
(d) Such high temperature increases the binding energy of the helium nucleus that is produced, making the fusion energetically possible.