Most importantly, aside from Zarzuela's scheduling decisions, Felipe and Letizia seem to enjoy working together.Agree. Zarzuela had wanted to find a balance that the heir would not be overshadowed by his wife, at the same time the Kings wouldn't be overshadowed by the Princes (this had been mentioned at Queen Sofia's recent book too).
I think the other main reason they had so many joint events is because they were very comfortable working together and obviously Felipe prefered this way (working together).