Usually it is assumed that in PIE the verb forms for the singular first, second, and third person are respectively -m-, -s-, -t- (cfr. Latin).
The personal pronouns, instead, have the second and third person inverted.
Why is this the case? I am no expert, but I thought that personal endings could have formed by agglutination with some forms of personal pronous.
My question is very much related to this question.