About when people start using plain form with each other
 

I have always wondered about this. I would imagine that when two people first meet, they would of course use polite form, like -masu and desu after their sentences. So when does the transition from polite to plain form happen? Does one of them just determines that "we are now close enough" and start talking to plain form? And then the second person follows suit? Or is it just depending on the person, like, maybe younger people are more casual so they will already start off using plain form (at least with people that are around their age)?

It don't just happen like that. It is not as simple as how you put it i.e. a "polite" to "casual" speech transition. One could be working together as co-worker for a year or two (or even more) and still be on the polite side. Same for the bowling mates that you meet every weekend.

The rule is to adhere to the senior/junior relationship closely. This is where you separate yourself between speaking polite or casual. And this is just part of the rule. There are plenty more.

I don't get why it is so hard to know when. Isn't it similar to English? You speak a certain way when speaking to people not your friends, people above you and those below you. You can tell how you need to speak depending on who your speaking to in English why is it so hard to do the same for Japanese. Is it because Japanese has different words for polite and informal? Same can go for English we use certain words with some and not with others.

Actually, it's not similar to English. Japanese makes greater use of honorifics. There are honorifics for nearly everyone a person might encounter, and not using the proper honorific can be offensive. The same goes for formal and informal language. In Western society, we tend to let it slide a bit more, but in Japan, it can be very offensive.

Then maybe its just me and the way I treat people or how I think they should be treated. I don't tend to let things slide.