I don't actually know the reason why theymos hasn't approved his application but I believe this user is quality enough to have a source which he would be that willing to be distrusting to community which he wouldn't see it as tasks to something to be rewarded thereafter. As I know to become a merits source is a self willingness to how people think of rending help to community which comes with not benefits just a dedication to the community and the true vision we have towards the development. As a matter of fact we are the preachers of Bitcoin and it's related branches which from the users around can actually feels the dedication and commitment.
However, I do support your application with all my power and good luck.
theymos has an overview of the statistics of the entire forum, including the number of posts and the number of merits shared, and based on that he concludes whether there is a need for a new merit source, there is nothing personal about it (whether he likes a certain member more or less).
If the need for a new merit source arises, due to the reduced volume of merits shared, I believe theymos checks the current applications for the merit source and selects the most qualified candidates.
Approval of new merit sources is generally not automatic, but always awaits a decision, depending on the situation on the forum.
In any case, I wish the OP good luck in applying for the merit source.