Why does Safari log me out of websites ?

About once a week, Safari forgets all of my log-ins (iPad Pro / iOS 9.3.2), and I have to go back to every website, and log-in all over again. I then might stay logged-in a couple of days, maybe a week, maybe longer, but then for no reason, I'll realize that I am no longer signed in anywhere. This is really a pain and quite annoying. How do I get Safari to remember me all the time ?