c# - button Onclick and checkbox oncheckedchange

I have 2 textboxes with compare field validator. I have a checkbox with oncheckedchange and button with onclick option. Clicking the checkbox or button should validate the textboxes.

1) When the page loads for the first time, if I click on checkbox, I get validation error as expected. Once I enter the valid data in 2 textboxes and uncheck the box, the page load function is called and it clears up entire data in the page and is not calling oncheckedchange event. why? 2) But if I do the same for button click, after page load, the onclick event is called.