atarasii kubiwa to riido wo katta node, inu mo uresisou desu.

Because 首輪 と リード is one combined object: collar and leash.

Japanese often links nouns with と and then marks the whole list with one particle:

• 首輪とリードを買った
• I bought a collar and a leash

You do not need:

• 首輪をリードを買った ✘

So と joins the nouns, and を marks the entire noun phrase as the thing that was bought.

In this sentence, most readers will understand 新しい as applying to both:

• 新しい首輪とリード
• a new collar and leash

This is a very common shorthand in Japanese, just like English can say a new collar and leash without repeating new.

If you wanted to make it completely explicit, you could say:

• 新しい首輪と新しいリード

But in natural everyday Japanese, repeating it is often unnecessary.

• 首輪（くびわ） = collar
• リード = leash / lead

首輪 is a native Japanese word written with kanji. リード is a loanword written in katakana.

So the sentence is talking about buying two different dog-related items: a collar and a leash.

ので means because or since.

So:

• 買ったので
• because I bought (them)

The whole sentence is roughly:

• Because I bought a new collar and leash, the dog also seems happy.

Compared with から, ので often sounds a bit softer, more explanatory, and slightly more polite.

For example:

• 買ったから = because I bought them
• 買ったので = because I bought them / since I bought them

Both are possible, but ので has a gentler tone.

This is normal Japanese.

The sentence mixes:

• a plain-form clause before ので: 買ったので
• a polite ending: うれしそうです

This is very common. In subordinate clauses like the one before ので, Japanese often uses the plain form even in otherwise polite speech.

So:

• 買ったので、犬もうれしそうです。 ✔ natural
• 買いましたので、犬もうれしそうです。 ✔ possible, but stiffer / more formal

The version in your sentence sounds natural in everyday polite speech.

も means also, too, or sometimes gives the nuance of even depending on context.

Here, 犬も suggests that the dog is happy too.

That usually implies someone else is happy as well, often the speaker or the owner. The idea is something like:

• I bought a new collar and leash, and the dog seems happy too.

So も adds a feeling that the dog's happiness is part of a bigger situation.

Without も:

• 犬はうれしそうです
• As for the dog, it seems happy

That would sound a little more neutral and less like the dog too.

うれしそう means looks happy, seems happy, or appears pleased.

It does not directly say the dog's internal feelings as a fact. It describes how the dog appears from the outside.

Base adjective:

• うれしい = happy, glad

Pattern:

• うれしい → うれしそう

So this sentence is not saying the dog is definitely happy in an absolute sense. It is saying the dog looks happy.

When そう is attached to an い-adjective to mean looks/seems..., you remove the final い first.

Pattern:

• うれしい → うれしそう
• おいしい → おいしそう
• たのしい → たのしそう

So:

• うれしいそう ✘
• うれしそう ✔

This is a very important adjective pattern in Japanese.

Because うれしそう can be followed by です in polite speech.

Even though うれしい is an い-adjective, once it becomes うれしそう, it behaves like a な-adjective-style word in many contexts:

• うれしそうです = polite
• うれしそうな犬 = a dog that looks happy
• 犬はうれしそうだった = the dog looked happy

So です here simply makes the sentence polite.

You could say that grammatically, but it means something different.

• 犬はうれしいです = the dog is happy
• 犬はうれしそうです = the dog looks happy

With animals, other people, or anything whose inner feelings you are judging from appearance, そう is often more natural because you are describing what you can observe.

So うれしそう is a very natural choice here.

It is the main topic/participant in the second clause, but Japanese does not always map neatly onto English grammar terms like subject.

What matters most here is the particle:

• 犬も = the dog too / the dog also

The first part gives the reason:

• 新しい首輪とリードを買ったので = because I bought a new collar and leash

The second part gives the result or observation:

• 犬もうれしそうです = the dog seems happy too

So 犬 is the thing being talked about in the second clause.

Yes, and the meaning would be very similar:

• 新しい首輪とリードを買ったから、犬もうれしそうです。

This also means Because I bought a new collar and leash, the dog seems happy too.

The main difference is nuance:

• から = more direct, more conversational
• ので = softer, more explanatory, a bit more polite

In this sentence, ので fits very naturally.

Both are possible, but they are slightly different.

• うれしそう = looks happy
• 喜んでいる = is showing joy / is delighted / is rejoicing

うれしそう focuses on appearance. 喜んでいる sounds a bit more direct and active.

So:

• 犬もうれしそうです。 = The dog looks happy too.
• 犬も喜んでいます。 = The dog is happy too / The dog is showing delight.

The original sentence is gentle and observational.

Yes, completely normal.

This sentence includes all three scripts:

• 新しい、を、買った、ので、も、うれしそう、です = hiragana
• 首輪、犬、買 = kanji
• リード = katakana

Japanese writing regularly mixes these scripts in a single sentence.

The spaces are probably just for learning readability.

Normal Japanese is usually written without spaces:

• 新しい首輪とリードを買ったので、犬もうれしそうです。

Textbooks, children's materials, and learner materials sometimes add spaces to make the sentence easier to parse.

So the spaced version is fine for study, but not how you would normally write it in everyday Japanese.