Wherein I Attempt to Justify Ill-Advised Behaiour on the Part of Fictional Characters

In Sasameki Koto, Ushio wants a girlfriend badly.

You'd expect: Ushio looks for an LGBT group nearby (nowadays you can find them in towns all over Japan) and/or uses the Internet. Since she's incredibly pretty and very friendly herself, she would probably not have much trouble finding somebody that way, even in heterosexist Japanese society.
Instead: She loudly goes after girls at her school, making them think that she's some kind of idiot.
Justification: Ushio is in fact in love with her friend Sumika (who's hiding from her in an increasingly shoddily-built closet) and uses her crushes to fuel her severe emotional problems that prevent her from acting on this out of fear. Also, she's in high school. She has plenty of time to meet girls the old-fashioned way, and since most of her peers already know that she's gay, what's the point of being shy?

In Harry Potter, Voldemort has a set of magical objects keeping him immortal.

You'd expect: Voldemort makes them impossible to discover: a pebble in the Gobi Desert, for instance, or a nondescript tree in a forest in Albania (where he's canonically spent a lot of time).
Instead: He uses symbolically important artefacts and hides them in places with personal relevance.
Justification: Voldemort is not a master planner. He is a vain, solipsistic psychopath.

In The Lord of the Rings, the characters have to undertake an arduous journey to destroy the Ring.

You'd expect: Gandalf asks the eagles from the Misty Mountains to drop them off in the Emyn Muil or somewhere.
Instead: They undertake said arduous journey on foot.
Justification: Sauron can very, very easily monitor Middle-Earth's airspace, which under normal circumstances doesn't have much flying in it other than the normal birds of the air and his own Nazgul. If the eagles left the Misty Mountains, he would know that something was very wrong.

In Kara no Kyoukai, Cornelius Alba is given Aozaki Touko's severed head and told not to destroy it because otherwise she'll be able to reincarnate into a purpose-built magical puppet, with all of her memories intact inculding those of the severed head while Alba is holding it.

You'd expect: Alba preserves the head in formaldehyde like the person who had it before him was doing.
Instead: He screams insults at the head, including the one insult that Touko has a proven track record of killing people for calling her. Then he smashes it and starts beating up one of Touko's employees, knowing full well that Touko is like a mother bear when it comes to them. Sure enough, she comes back and kills him horribly.
Justification: By this point Alba is batshit insane and also doesn't know that Touko has the ability to reincarnate. However, this only raises the further question of why he never bothered to find out, considering that by this point he and Araya have been planning to attack Touko and her group for some time.

In The Adventure of the Devil's Foot:

'Hey Watson, look, a hallucinogenic drug that drives people insane and then kills them! ...LET'S TEST IT ON OURSELVES.'
Justification? No, for some things there's just no excuse. Sorry, Holmes.