Winforms c# - Set focus to first child control of TabPage

Question

Say I have a Textbox nested within a TabControl.

When the form loads, I would like to focus on that Textbox (by default the

Answer 1

The following is the solution:

private void frmMainLoad(object sender, EventArgs e)
{
    ActiveControl = textBox1;
}

The better question would however be why... I'm not entirely sure what the answer to that one is.

Edit: I suspect it is something to do with the fact that both the form, and the TabControl are containers, but I'm not sure.