This is because you are not telling f2 who its Parent is.
If this is an MDI application, then f2 should have its MdiParent set to f1.
Form f2 = new Form() { Width = 400, Height = 300 };
f2.StartPosition = FormStartPosition.CenterParent;
f2.MdiParent = f1;
f2.Show();
If this is not an MDI application, then you need to call the ShowDialog method using f1 as the parameter.
Form f2 = new Form() { Width = 400, Height = 300 };
f2.StartPosition = FormStartPosition.CenterParent;
f2.ShowDialog(f1);
Note that CenterParent does not work correctly with Show since there is no way to set the Parent, so if ShowDialog is not appropriate, the manual approach is the only viable one.