Who's read the old Calvin and Hobbes comics where Calvin's dad says "It builds character" whenever Calvin is complaining? I have, and I always found them hilarious.
Turns out it's true: hardships do build character. Not necessarily mosquito bites, but other things, much worse things. Look at your own writing, or if you don't have anything bubbling in the cauldron right now, one of your favorite books. What hardships does the main character (or any character) endure? How does it change him? I know this sounds more like a literature question than a writing question, but think about it.
A better question, if the thing bubbling in your cauldron is a little flavorless, is how do you want your character(s) to change? And what hardships would they have to endure to get there? Generally, once you know the answer to the first question, the answer to the second isn't too hard to find. Just look at your character's personality. What do they resist? Who or what do they love that perhaps they shouldn't? These questions are vital when it comes to forcing your characters to adapt.
I'll go first. One of my characters thinks she's a hero until she loses her magic. Then she thinks that she's pretty much useless without it. And then she still finds ways to help another character, because surprisingly she has more skills than just magic. Or take that other character, for example. She's rather snotty and disagreeable when we first meet her. I'd prefer for her to become a little less bratty over time. How can I make her less selfish?