Problem with ngOnInit in ionic v4

hello, i have made two pages,first one is home page and another is cities page, firstly home page is loaded and ngOnInit is called and on click event of home page i navigate to cities page, but when i am navigating from cities page to home page it again call ngOnInit of home page, i am confused with ngOnInit flow can anyone help me, why ngOnInit is called everytime when i am navigating from cities page to home page.