Why would one change from using polite form (です/ます) to using the plain form in the middle of a conversation (and change back later)?

Question

I am reading The Curious Incident of the Dog in the Night-time by Mark Haddon in Japanese.

Towards the end of chapter 67, Christopher is talking to Mrs Alexander, trying to find out who killed Wellington the dog.

I noticed that Christopher started off by speaking politely, using です and ます, but then he says something that ends in る. I am very confused by this.

Here is part of the conversation. Note that I removed all the other non-dialogue text and C for Christopher and A for Mrs Alexander.

A: あなたのうちには犬はいないでしょ？

C: ええ

A: あなたはきっと犬が好きなのね？

C: ネズミを飼っています

A: ネズミ？

C: トビーという名前です

A: おや

C: 大概の人はネズミを嫌がります、なぜかというとネズミは腺ペストをうつすと思っているから。でもそれは彼らが下水管に住んでいたり、珍しい病気のある外国からやってくる船にこっそり乗り込んでくるせいです。でもネズミはとても清潔です。トビーはいつも自分の体をきれいにしている。それからトビーをわざわざ散歩に連れて行く必要もない。ぼくの部屋にはなして走りまわらせて、運動させるだけでいい。それからときどきぼくの肩に座ったり、巣穴のつもりでぼくの服の袖に隠れたりする。でもネズミはかならずしも巣穴には住みません

A: なかに入ってお茶を飲まないこと？

C: 他人の家には入らない

I have made all the polite forms bold and all the plain forms italic. Christopher is starting with ネズミを飼っています and then changes to plain form here: トビーはいつも自分の体をきれいにしている. He then continues with the plain form until this: でもネズミはかならずしも巣穴には住みません. And then changes to plain form again here: 他人の家には入らない.

Note that I understand that plain forms should be used in a relative clause like 珍しい病気のある外国からやってくる船, I'm talking about why he ends his sentences with plain forms.

I have never seen this kind of thing before. Characters in other books I read don't seem to do this. The forms they use seldom change and changes are few, and never more than twice in a single dialogue! Why does Christopher switch between the two politeness levels?

My guess is that this is a symptom of Christopher's autism. Because he is autistic, he mixes up the two forms.

Answer 1

It's quite natural to mix forms. While we are taught as beginners that there is just a change in register, there's a lot more going on. E.g. the plain form could in some contexts be more formal than the desu/masu form as it increases distance between the speaker and the listener. The cat in 吾輩は猫である sounds pompous for this reason.

However, the plain form can also be more emotive. Desu/masu implies a more factual statement, and the plain form can be used for things closer to the speaker.

Look at the sentences with desu/masu:
でもネズミはとても清潔です But mice are very clean
でもネズミはかならずしも巣穴には住みません But mice do not necessarily live in holes
These are statements of fact not directly related to Christopher.
In contrast, look at the plain form sentences:
トビーはいつも自分の体をきれいにしている Toby is always cleaning himself.
他人の家には入らない I don't go into other people's houses.
These are all things that are closer to Christopher.

Plain and desu/masu forms carry far more nuance that just degree of politeness.