"Now that" and "since" can both mean "because". "Now that" is more related to TIME when it has the meaning of "because" and it is closer in meaing to "seething that". "Since" is closer in meaning to "because" when used in that way. Now that you are here, we can leave. but Since you don't like rice, what will you eat in China? "Now" can be used alone without "that". Now you're here, you can help me. Don't forget that "since" has a second meaning related to time that does not mean "because". The second meaning of since is " from a time in the past when". Example: Ever since I was a child, I loved to read about China. :)

"Since" is used in the sense of "because". It implies a condition that is a reason for a result. "Now that" implies the recent occurrence of some happening that allows a result. "Since you don't like action films, let's see a comedy" There is no occurrence here, but a condition of not liking action films. "Now that you have your drivers license, you can drive my car." This means that because something happened recently (getting your license) a result follows (driving my car).