Why is ‘ni’ used in these sentence...?

Particles are a tricky subject for me...
I was reading in my language textbook and I came across two sentences as follows: "Isha ni nari-tai desu"(I want to become a doctor), and "Okii ie ni sumi-tai desu"(I want to live in a big house). I don't mean to come off as stupid, but I really don't understand why 'ni' is used. Can someone explain?

Some basic meanings of the particle "ni" are: (1) sense of direction toward something, and (2) indicating a place where you are/something is.

(1) "... ni naru/narimasu" simply means "to become (something)," "turn INTO (something)," as there is a change from what it is now TO or TOWARD what it will change to. In "Isha ni naritai desu," (of course you don't say "I want to become to a doctor," but) there is the change from what you are now TO a doctor.
Other examples using "ni" in this meaning are:
- Mainichi gakko ni ikimasu. (I go TO school everyday.)
- Tomodachi ni furui shashin wo misemashita. (I showed old photos TO my friend.)
- Anata ni aitai desu. (I want to see you.) In English there is no "to" or other preposition, but there is clearly a direction toward "you."
- Koneko wa 2-nen de okii neko ni narimashita. (My kitten became/turned INTO a big cat in two years' time.)

(2) In the other sentence, "Okii ie ni sumitai desu," it's simply like "I want to live IN a big house," indicating the place where you will be/reside.
- Gakko ni toshokan ga arimasu. (There is a library AT/IN the school.)
- Fujisan wa Nihon ni arimasu. (Mt. Fuji is IN Japan.)


There are a few other meanings that "ni" carry, which include "purpose" (Ex. "Kaimono NI ikitai desu. - I want to go shopping) and action done BY someone ("Sensei ni okoraremashita. - I was scolded BY my teacher.) but those will come gradually :)

Thank you so much! That was really helpful :)