Because people want your stuff, and it is really easy to take it unless you make it hard. Do you want to know the most common password out there? 123456. Followed swiftly by 123456789, qwerty, password, and to round out the top 5 most common passwords 12345. Please don't use these, they are the first ones that thieves try.
In this beautiful digital age, they have programs written and easily available to crack passwords. These programs do what is called a brute force attack. They just run through every possible combination of letters, numbers, and characters until they get it right. Which is why every website wants you to make it long and ugly. The longer and uglier it is, the longer it is going to take for someone to crack it. For example, 062116jh takes about 4 minutes, my-fair-lady takes about 3 hours. Make it my-fair-lady* thats about a week. my-fair-lady*1 now it's a little over a year, about 13 months. It would take them 25,947 years to get to my-fair-lady*198. If you really wanted to tick them off go for something like my-fair-lady-@4598 because that will take about 116,477,121 years. By that time you should have had to change your password at least as many times. I recommend changing passwords every 6 months, just to be extra safe, but once a year works just fine.
How are you supposed to keep track of all these passwords with all these requirements? The way I recommend the most is to find a password keeper app. I like Keeper. Pros: It will hold all of your passwords. It is password protected, if whoever is trying to get in tries too many times then it will delete all the information stored in the app. This is true for many password storage apps. They also have family plans. Cons: it does require a paid subscription, however they do have a free 30 day trial so you can test it out and see if it will work for you.
Sources:
https://cybernews.com/best-password-managers/most-common-passwords/
Android Phones: the Cool Stuff, presentation by Administrative Freedom LLC
