R’ Moshe Feinstein zt”l (Darash Moshe) would say: “"והלוי אשר בשעריך" - Why does the posuk mention ‘the Levi in your midst’ specifically in regard to the holiday of Shavuos and the simcha associated with that holiday? Why not by any of the other holidays mentioned in the posukim? The Gemara notes that everyone agrees that on Shavuos you need ‘Lachem.’ Now, for a Levi and other Talmidei Chachamim this makes good sense, for after all, he is involved in learning. But what about the person who is working? Thus, the Torah tells us to take him - the Levi - into our midst and make sure the joy begins in the right place - and go from there.”
A Wise Man would say:
“Nothing can disturb your peace of mind unless you allow it to.”