SessionID keeps changing in ASP.NET MVC why?

I've seen that happen even without MVC. If I remember correctly, ASP.NET keeps assigning new session ids until you place something into the Session variable.

You should initialize the Session object within Global.asax.cs.

void Session_Start(object sender, EventArgs e)
{
    HttpContext.Current.Session.Add("__MyAppSession", string.Empty);
}

This way the session will not change unless your browser window is closed.