unusual behavior with strings append property c++

   #include<iostream>
#include<string>
#include<algorithm>
using namespace std;

int main()
{


  string a ="hello";
  string b = "dear";

for (int i = 0; i < 3; ++i)
{
        string x = a.append(b);

  cout<<x<<" "<<b<<" "<<a<<endl;

}

}

why value of a changing i am just changing x

output::

hellodear dear hellodear

hellodeardear dear hellodeardear

hellodeardeardear dear hellodeardeardear