There's nothing worse for an introvert than running into someone they know from high school. No matter how many times they avert their eyes and try to escape, the high school acquaintances will never get the hint, insisting they rehash the awkward memories from their teenage years, like acne, getting picked last for dodgeball, and the one time that one cheerleader got pregnant. Introverts don't want to dwell on the past, especially from their uncoolest years.
And even though we're sitting on the same public transport in the same direction to a similar place, we don't need to yap about life.
We don't need to "catch up," we don't need to fill the silence, and we certainly don't want to exchange contact information. Perhaps I'm antisocial like that, but my two friends are all I need in this world. I'm already at my social limit with my tight-knit crew, and because of that, many introverts have no desire to add one more name to the list of people they need to remember to text back.