Altering objects in Unit Testing?

I'm trying to test a class. Doing so, I'm changing the objects state, yet for some reason, that change is only temporary.

Here is what I mean:

class ownerTest {

Owner Tom = new Owner(); // creates owner and initializes arraylist of type pet
Pet Doggy = new Pet("Rexy"); // creates pet object doggy

@Test
public void test1() {
    Tom.addPet(Doggy); //will add doggy on to tom's arraylist
    assertEquals(Doggy, Tom.petList.get(0)); // passes
}

@Test
public void test2() {
    assertEquals(Doggy, Tom.petList.get(0)); // fails cause arraylist is empty
}

I've been told that I can treat test classes much like normal classes. Then why are changes made to objects in test units only temporary?

Would love some clarification.

PS: The equals method has been adjusted accordingly so it's not about that.

Thanks in advance!