What quality does the speaker praise in sonnet 116?

Sonnet 116 - Let me not to the marriage of true minds - praises constancy in love (it is an ever-fixed mark /That looks on tempests and is never shaken;).

Sonnet 116 is a sonnet which is more radical than it looks (the Beatles' She Loves You broke several conventions of contemporary lovesongs, but so subtly that few people noticed).

Shakespeare praises love for its constancy and enduring qualities, at a time when almost all other sonnets focused on how exciting and in the moment sudden pashes were. Shakespeare also talks about admiring love in other people, at a period when love sonnets almost always focused on the love the poet himself (always himself) felt.