Happiness is a circle that's tenacious. In other words, we're all in this circle/cycle where we do whatever we can to grab onto happiness and hold it as tightly as we can.
Happiness is as tenacious as a shih tzu (しつ) with a shoe (しゅう). That little dog does not want to give up on the shoe, no matter what's happening around it.
Currently there are no similar looking kanji which exist in the WaniKani learning system