The main reason to veto the Reform is: Power. Since 1945 until today, The Permanent Five (P5) rejected all kinds of reforms and even don't accept to share the power. Several proposal were introduced to include the expansion to include more permanent members.
The most popular is the G4 (India, Germany, Japan, Brazil), which want to include seats for Western Europe, Latin America, Africa and one more seat to Asia. To pass the reform they need the approval of the P5 plus the backing of the two-third of the UNGA. The hard part is the China's agreement; remember that China historically has several issues regarding their foreign policy with Japan, the last was the Senkaku controversy.
Another proposal is the one made by the other G5 (Argentina, South Korea, Pakistan, Mexico and Italy); the support the creation of new rotative permanent chairs. For the same reason of the G4's rejection, I think this one won't work either.
Kofi Annan several years ago proposed to expand the number of permanent seats, but without the veto power and more permanent seats but this was rejected by the regional blocks.
There's no reason why the P5 want to accept one more permanent member. Even if the growth of the regional power, still not being enough reason to add more permanent members with veto power.