In studying the martial arts, in order to improve it is absolutely essential to spar with your betters. There is no way around it: if you want to get better, you have to spar with partners who can hand you your ass on a platter. Mind you, the lesson is not to just stand there and take a beating, but to ultimately become strong and skilled to the point where they can no longer defeat you.
One doesn't grow in times of ease and comfort: we need Challenge to progress. One doesn't become a better musician by only tackling easy pieces. One doesn't become a better athlete or scholar or anything else by only following the easy path. One doesn't become better at anything by staying close to the shore of what is comfortable — growth requires that we sail out into the Terrifying Deep.
That can certainly be applied to the Spiritual Path as well: Jesus didn't say "Take up your sofa and follow me."
So a Worthy Opponent is someone or something that forces you to step up to the plate and grow. Consequently, I think it is in fact a great blessing to have a Worthy Opponent in one's life — whether that be a person or a circumstance.
Interestingly, the word for 'Satan' is Hebrew for "Adversary". This from Wikipedia:
The original Hebrew term sâtan (Hebrew: שָּׂטָן) is a generic noun meaning "accuser" or "adversary", which is used throughout the Hebrew Bible to refer to ordinary human adversaries, as well as a specific supernatural entity. The word is derived from a verb meaning primarily "to obstruct, oppose".Wikipedia: Satan
Be agape-loving your enemies, be doing good to those hating you. Be speaking well of those cursing you, Be praying for those mistreating you. (Luke 6:27-28)
And why, in essence, should we bless them? Because they are blessing us — by providing an opportunity to become stronger and more skilled at managing the hardships of climbing the Sacred Mountain.