Translation:To love and to be loved is the greatest happiness.
From "Nature Boy" the fabulous song by the first 'hippy" Eden Ahbez. in, believe it or not, 1948 way ahead of his time. He lived under the Hollywood sign in Beverly Hills...in a tent and rode a bike everywhere. He sought harmony with nature, ate a vegetarian diet, believed in free sex, and the use of hallucinogens.
This song has been covered by a galaxy of stars. As Bioem says it goes "just to love and be loved in return"
Considering that this sentence has been up for 6 years and there have been very few complaints about this, I am inclined to suppose that a great many people did get answers with "aimé" accepted, and that there was some other error in your answer, maybe just a small typo.
I do find that Duo is quite vague about what was wrong with a rejected answer. The answer Duo gives instead is often quite different in many ways from the rejected one, but most of the differences are not errors, just other accepted possibilities. You really have to look to see what it might actually be objecting to. It's confusing.