There might be a variety of reasons why any sane adult would willingly give up a couple hours of their time every week to attempt to disciple middle school and high school students:
Teenagers say and do some pretty goofy things. Perhaps some people volunteer because they love to laugh.
Teenagers have a tendency to say and do some pretty weird things. Perhaps those who volunteer are insane.
Teenagers like to do and say some odd things. Perhaps the volunteer has an agenda to fix an entire generation and developmental category. Good luck.
Need To Be Liked
Well… teenagers say and do some awkward things. Perhaps if a volunteer needs to be liked by someone and they think a “simple” young teenager can fill that void, well… that’s just awkward.
Share the Gospel
Teenagers don’t seem to have control over what they say and do. Perhaps adults volunteer in youth ministry because they believe that God loves all people, uncontrollable teenagers included, and desires to save all people, awkward teenagers included, and that living life in dependence on the Holy Spirit is possible for all people, odd teenagers included, and no agenda, desire to be liked, insanity or fodder for laughter will preclude weird or goofy teenagers from the Gospel of Jesus Christ.
That’s why you should volunteer!