It wasn't because Elsa was very hard to deal with, but because the iron lady was too intimidating and most male detectives didn't want to taste being suppressed by her aura.
Luke and Selina were the only two in the Major Crimes Division who might be Elsa's subordinates in name but were actually more like her friends.
Luke had set an example by joking with Elsa every day. Naturally, Selina wasn't scared either. So, the relationship among the three was very natural.
Thus, the backyard that afternoon became a playground for the three women. Luke simply set the grill to keep the food warm, before he went back inside to work.
Since he couldn't tan, he was only keeping Selina company during the occasional moment that he sunbathed.
Since she had someone else to keep her company today, Luke decided to go to the basement and tinker around with his own hobbies.
