Whenever someone states a job or person they're with is not "challenging" what they are really saying is they are "bored" to death!
I have seen "nice guys" wonder why women chase after the "bad boys" and the answer is they see them as being a "challenge", someone who they have to win over, compete for, (earn) their attention or love.
I've often said you can stick a woman in a room with 5 guys and have 4 of them get on their knees extending their heart towards her while the fifth guy sits in a corner sipping away on a cocktail and acting like she does not exist....And that will be "the guy" she wants to get to know!
"We ignore those who adore us and adore those who ignore us."
Some people want to be "challenged". Personally speaking I'm a "no drama" kind of guy.
A few years ago there was a bestselling book:
"Why Men Marry Bitches: A Woman's Guide to Winning Her Man's Heart by Sherry Argov"
Apparently there are some men and women who are excited when they don't know where they stand with their mate or when it's difficult to make her/him happy. Drama is their "friend". They love the thrill of being on a relationship roller coaster ride. For them words like lovable, dependable, reliable, giving, nurturing, steady, balanced, and predictable means BORING!
While on the other hand there are people who view those same things as making them feel intimacy, connected, and emotionally secure within the relationship. It always comes down to finding someone who wants what you want. Some people are only excited when things are "new". They have no interest in the "tried and true".