Loony Friends Improve Your Personality

A stiff character learns and grows from unwanted interaction with annoying and eccentric people.

The focus character is a fairly stiff guy (rarely a gal), may be the Only Sane Man, The Comically Serious or a Jerkass of the "stick up his rear" type, and often comes across as an Ineffectual Loner. He may not be content with his life, but it's stable, and he probably has a long-term plan for fixing what he thinks is wrong, if he can just get the right breaks. Instead, he is dragged into wacky hijinks by the other characters against his will, making a mess of his life. But implicitly or explictly, the goal in the story is to make him a better person by putting him through horrible experiences.

May involve a Manic Pixie Dream Girl, but in most examples, it's an entire cast imposing on the stiff guy's time, money and patience. The Power of Friendship is usually evoked by the end of the story. There may be a montage as the character remembers how things used to be, and how despite how much the other characters irritate them, they have made stiff guy a better person.

Contrast Unsympathetic Comedy Protagonist who may have a similar personality and equally annoying cohorts, but does not have the Character Development.

Anime and Manga

 * Rozen Maiden: Jun is a Hikikomori until Shinku forces him to be her "medium", interacting with her and the other crazy dolls and eventually humans brings him out of his shell.
 * Gash Bell: Kiyomaru, who was hiding out at home until Gash forced him to interact with people.
 * Midori no Hibi: Seiji thinks that his problem is not being able to get a girlfriend, but that's actually just a symptom of his general inability to interact non-violently with others. Having to protect Midori and her secret, and deal with wacky people he can't just punch, brings out his better qualities.
 * Explicit in Haunted Junction: Hayato, who says I Just Want to Be Normal, but is constantly dragged into supernatural mayhem. In the final episode we learn that the school spirits have been tormenting him specifically to avert a Bad Future in which he lives a "normal life" but never emotionally connects with anyone including his future wife and child. Which seems to lead into a happy ending...except that said Preacher's Kid was made so miserable by the constant torment that he voluntarily participated in a plan to kill or imprison all the school spirits; and there's no indication that things are going to get better for him.
 * The plot of Happy Lesson kicks off when Chitose's teachers decide that what he really needs are five Hot Moms running his life. (He discovers, much to everyone's surprise, that he does become a more well-adjusted person because of them.)
 * Sumomomo Momomo: Koushi never wanted an Arranged Marriage or to be surrounded by lunatic martial artists bent on killing/loving him. But after Momoko finally admits she knows he doesn't love her, Koushi acknowledges that his experiences have broken him out of his fear of confronting other people (even if he's still a physical coward.)
 * The residents of Maison Ikkoku spend most of their time driving Yusaku Godai insane with their alcohol-induced insanity, but also end up somehow making him the man he needs to be for Kyoko, as well as get Kyoko to reflect on her feelings for him.
 * The Melancholy of Haruhi Suzumiya implies this is the case for Kyon, who became a much more outgoing and likable guy after exposure to the SOS Brigade. It inverts it with Haruhi, who became a more sympathetic person by hanging out with apparently normal people.
 * Arakawa Under the Bridge: Ko/Recruit starts the series as someone who sees all human interaction as a series of debt transactions, in which he must never remain the debtor. Being forced to live with the zany riverbank cummunity allows him to learn to interact with other people on the basis of honest emotion, without being concerned about who "owes" who. Lampshaded when Recruit realizes he hasn't had an asthma attack (symptomatic of his fear of debt) in quite some time.

Film -- Live Action

 * Tim from Dinner for Schmucks after hanging around with Barry.
 * Claude from Hair (theatre) epitomizes this trope.
 * The Fisher King. The "stiff guy" is a shock radio host, but he's still quite stiff. And the looney is...well...Robin Williams. As a crazy homeless guy. It's a surprisingly serious movie, but still, you can see where this is going.

Live Action Television

 * Jeff Winger in Community grows (slowly and with much backsliding) from Jerkass to Jerk with a Heart of Gold when forced to interact with his unwanted study group. See especially the episode "Paradigms of Human Memory."
 * Deconstructed in "Studies of Advanced Movement", when Annie—a similarly stuffy and uptight character (while hardly the Only Sane Man) -- is moving in with Troy and Abed, the show's primary source of wacky hijinks, and is advised to be flexible with them. After an increasingly trying and stressful first day of putting up with their antics, however, she snaps and delivers an angry lecture about how unfair it is that she always has to be the one to adapt to their personality quirks while they seem to show no intention of attempting to accomodate her.
 * This is the basic plot of Producing Nobuta.

Western Animation

 * Twilight Sparkle in the premiere of My Little Pony Friendship Is Magic is actually made a better pony of sorts when forced to make new friends, some of whom are definitely bizarre.