Start with these premises:
- Value personalism: Nothing is more valuable than a good person except perhaps for another good person.
- If A has finite value and B has positive value, and the mereological sum A+B exists, then the mereological sum A+B has more value than A.
So, if there are mereological sums, finite persons are not a part of them, unless the sums themselves are persons. But it would be weird if it were possible for finite persons to be proper parts of mereological sums that are persons but not of other mereological sums. So it is reasonable to conclude that finite persons can't be proper parts of mereological sums. But neither can God be a proper part of any mereological sum. So, no person can be a proper part of a mereological sum.
I think the best explanation of all these facts is that there can't be such a thing as a mereological sum.