Sun goddesses and their hair care products, amirite fellas!?!?
At least it specifically said "no more tears" on the bottle, hopefully not just because they've all evaporated.
Dunno what's up with Celestia's hair but I'm pretty sure it's not actually hair. But that's one of the neatest things about the design in this show - It woulda been easy to just give the Princesses traditional, if elaborate, physical manes, but a curtain of light is much more interesting and cool.
But it does look like it might mess you up if you touched it, so of course keeping it looking its best would require correspondingly "hot" shampoo.
Also, since you can see space through Luna's hair I subtly put the sun in Celestia's, and also to make it clearer she's partly made of magic, or something.