He was not one of the Twelve; but after the treachery and death of Judas Iscariot, someone was needed to take his place. Two candidates were selected, and lots were drawn to see which of them should be made one of the Twelve: the choice fell on Matthias.
When we attain some high or responsible position, we may be tempted to congratulate ourselves on being the best candidate for the job. We would do well to remember that we have got there because of the people we have met and the things we have found ourselves doing, and, because of the gifts and talents that God has given us.
These things are essentially random: like Matthias, we have been chosen by lot.