jmc

Among the two-element subsets of $\{1,2,3,4,5,6\}$, each element in $\{1,2,3,4,5,6\}$ appears 5 times, one time in the same subset with each other element. Thus, the desired sum is $5(1+2+3+4+5+6)=5\left(\frac{6\cdot 7}{2}\right)=\boxed{105}$.