I LOVE Mary/Sherlock friendship. While John is the most important, I feel sometimes we overlook how much Sherlock needs Mary too. She supports him, supports John continuing to work with him, and helps so much.
This was one of my favourite parts of The Sign of Three. Sherlock is so jealous of Major Sholto, and Mary as well. The funny part is that Mary not only sees the blatant jealousy, she teases Sherlock, not unkindly.