Strange behavior of a boolean variable, the value of the variable is lost between the postback

I have a boolean named CheckBoxActivated, which I set to true after checking the username and password.

string name = us.UserName;
string password = us.Password;

if (name.Equals(txtName.Text) && (password.Equals(txtPassword.Text)))
{
    CheckBoxAvtivated = true;

The strange thing is: after I return to the variable, I press another button and immediately becomes “false”, which leads to undesirable behavior.

protected void butNext_Click(object sender, EventArgs e)
{
    if (CheckBoxAvtivated)
    {
        pnlCheckBoxes.Visible = true;
        pnlUserCheckBoxValidation.Visible = false;
    }
    else
    {
        pnlCheckBoxes.Visible = false;
        pnlUserCheckBoxValidation.Visible = true;
    }

Thus, the state of the variable unexpectedly changes to false. Any reason why this could happen?