If you have watched the documentary Lucy and Desi: A Home Movie, you'll see clips of them acting childlike and silly when they were first married. They dressed in costumes, partied, teased and horse played, took adorable pictures and videos of each other, and thoroughly enjoyed the company of each other. I suppose that they were able to throw away their adult problems momentarily and return to the carefree childhood. Too bad they became the main source of unhappiness for each other 2 decades later and had to separate. People change; external circumstances never stay the same. However, Desi and Lucille loved each other in spite of all the mistakes... till the very end.
That magic between these two is the ingredient that makes I Love Lucy so successful.