Set vs List in REST One To Many

Question

I have one to many relationship. If in class Customer I write List:

private List<Orders> order;

my GetMapping will work fine. But I want to use best practices and I write Set instead of List:

private Set<Orders> order;

In result I have error:

Could not write JSON: Infinite recursion (StackOverflowError); nested
exception is com.fasterxml.jackson.databind.JsonMappingException: 
Infinite recursion (StackOverflowError)

Why I have this error? What's wrong with Set?

My entities:

@Entity
public class Customer {
    @Id
    @GeneratedValue
    private int id;
    private String firstName;
    private String lastName;

    @OneToMany(cascade=ALL, mappedBy="customer", orphanRemoval=true)
    private Set<Orders> order;
    //private List<Orders> order;
}

@Entity
public class Orders {
    @Id
    @GeneratedValue
    private int id;

    @JsonIgnore
    @ManyToOne
    @JoinColumn(name="customer_id", nullable=false)
    private Customer customer;
}

And GetMapping:

@GetMapping("/customer/{id}")
public ResponseEntity get(@PathVariable Long id) {
    Optional<Customer> customer = customerRepository.findById(id);
    return new ResponseEntity<>(new ResponseObject(customer));
}

UPD. I see question Infinite Recursion with Jackson JSON and Hibernate JPA issue. But it's other question. I talk about difference in use List and Set. I am not interesting in @JsonIgnore and I don't ask about it (and I use it in my code). I want to understand why I have an error when I use Set and don't have error with List