Why doesn't Ajax calendar extender work until I click any button ?

I have an AjaxCalendarExtender applied on text box. It doesn't work until i click button within page (any asp button which doesn't post back). After clicking any button it starts working like a charm.

Why ?

<asp:TextBox ID="txtBoxDateOfBirth" runat="server" CssClass="form-control"></asp:TextBox><asp:CalendarExtender ID="calExt1" runat="server" Format="dd/MMM/yyyy" TargetControlID="txtBoxDateOfBirth">

Button:

<asp:Button ID="btnShowViewPersonalInfo" runat="server"OnClick="btnShowViewPersonalInfo_Click"/>

this button doesn't nothing big, just changes VIEWs in webform.