The prophet Zachariya (3,7), according to the Targums explanation, refers to angels as "those that stand by".
'Thus saith the LORD of hosts: If thou wilt walk in My ways, and if
thou wilt keep My charge, and wilt also judge My house, and wilt also
keep My courts, then I will give thee free access among these that
The accepted reason that angels are called "those that stand by" is because they are created at a specific spiritual level and have no way to climb higher, as apposed to us humans who's life mission is to reach higher and higher.
Rabbi Yoḥanan is teaching us that one should only learn from a Rabbi who is similar to an angel in this aspect, that he is willing to halt his own self-growth in order to allow his students to grow.
I heard this explanation from Rabbi Yehoshua Heshil Eichenstein Rosh Yeshiva of Yad Aharon some 20 odd years ago.