I have a tendency to "organize" stuff in my brain and that's probably how I built my own personal/internal database of knowledge.
For example:
When I was young in the 70s I didn't know much about theatre - I had seen a National Tour of Annie (was stunned by it... my favorite songs were "Maybe" and "We'd Like to Thank You Herbert Hoover" (go figure.) and the only other stage productions I had seen had been on field trips to our local high school to see Funny Girl, Brigadoon, Godspell and Guys & Dolls. I was hooked. Hooked hard!

I watched the Tonys religiously from 1979 on.
By 1979 or 80, my sister went on a High School trip to New York and saw a few shows (Deathtrap, Sugar Babies, Evita and Sweeney Todd) She bought the cast albums of Evita and Sweeney and I had never heard of Andrew Lloyd Webber or Stephen Sondheim at that point. I remember pouring over those recordings every chance I got and became "obsessed." Before long, I knew every line in Evita (I guess it was kind of like my "Elphaba" era - Patti LuPone had been my Idina Menzel

) I also remember thinking: "Who is this Stephen Sondheim guy?" I had no idea that you could be shocked/scared by a musical! I started doing "research" into everything else Sondheim had written up to that point. Company! How edgy and exciting it was to a 12 year old kid! How could someONE write Sweeney AND Company?! When I discovered a composer I liked, I would find everything he/she had written and dig, delve and learn.
As time did the ol' marching on thing, I took an historically organized approach. I researched which shows won Tony Awards for best musical and learned all about each one. I'd buy a cast album sometimes twice a month and "study" it. Being a total research nerd (yes, even as a kid) I loved being able to rattle off the years and Tonys ...
From there, once I had gotten all caught up and current, I started to become interested in researching shows - both plays and musicals - that opened in the same seasons. So I would learn about each nomination (for best play/musical). I "discovered" Kander & Ebb, Cy Coleman, Jule Styne, Bob Fosse, Tommy Tune, and COUNTLESS others and made connections between not only the other shows with which they were involved, but then which performer was in what show, and then what other shows did that director/performer/composer/write do... and from there ... well, it's an amazing way to fuel my passion, to do that kind of research and make those kinds of connections.
To the original poster, I'd just like to suggest that perhaps you ought to think that if you enjoyed Wicked very much, start looking at everything else Stephen Schwartz has written. Maybe you'll find a new love with Godspell or Pippin, etc. Maybe you won't, but it make spark a curiosity around what else was playing at the time of Pippin (1973): Grease, Man of La Mancha, Purlie, A Little Night Music... Try out a couple of those. You may not like ANY of them, but you'll be expanding your knowledge considerably, just by following your curiosity.
(Sorry this is such a long post)
In terms of plays, you could do this much the same way. Have you ever seen a play? Did you like it? What was the name of the playwright? Get thee to library and read all of the other plays from that playwright, then expand by season or actor or director.
I admit that this approach may not work for you/everyone/anyone, but, as I said, my natural tendency to organize and research helped me fuel my passion for all things theatre. It's hard sometimes to feel the love for something so strongly but not know how to go about expanding it. This might be one way you could go about it.
~Adam
(PS: Do you think I felt awkward in any way, singing the score to Evita while at Boy Scouts summer camp?? .... pretty clueless, wasn't I?... But I actually got my fellow scouts to sing "Peron's Latest Flame" while we were hiking...)