Is it dangerous to use the same password for different accounts on the internet?

I have three different passwords that I use across the internet and one that i’ve used since 7th grade and use 90% of the time all usually with the same screen name for all of them. (minus fluther to keep some level of anonymity in case someone I know is on here and reads a question i would prefer they not read. which is one of the things i love most about fluther)

but anyway is it dangerous to have the same password for all of my different accounts?

1. because if someone finds out one they know them all.

2. can admins of websites get my password and use it maliciously to hack other websites i use?