Adam Clarke on 2 Corinthians 6:12

Ye are not straitened in us - That is, Ye have not a narrow place in our affections: the metaphor here is taken from the case of a person pent up in a small or narrow place, where there is scarcely room to breathe.

Ye are straitened in your own bowels - I have not the same place in your affections which you have in mine. The bowels are used in Scripture to denote the most tender affections. See the note on Mat 9:36.