I think my worst fear in life or at least one of them is an airline losing my luggage. Not only does it royally screw up your entire trip (if it happens on the way out), but what about all the things you'd have to replace? I keep that in mind when packing my carry on. I also pack it depending on how long the flight might be or what didn't fit in my checked bag.
Glasses and contacts are some of the most difficult items to get replaced on vacation. They require prescriptions and eye doctor visits and fittings. To avoid that, I always pack my contacts and glasses in my carry on.
Since the air gets dry while flying, I always make sure to have my Bag Balm lip balm and hand lotion. Advil, tissues, a hair tie and headband are also necessities.
I try to always take the smallest purse possible on vacation. So, I usually tuck my purse into my carry on. I have been loving my Penny Purse; it holds a surprisingly large amount of items for its size and it's so lightweight.
I also pack all of my non-liquid makeup in my carry on. That way, TSA doesn't stop me and if my luggage goes missing, I only have to replace a few makeup items and not the entire bag.
Lastly, I always have my sunglasses, any jewelry I packed, my iPad, iPhone and notebook. (Not pictured are my camera since I'm using it and all my charging cords and headphones.)
What items are necessary in your carry on?