As to the English question, it's the same reason that when you put 你 and 好 together, the tone of 你 changes from ni3 to ni2: to simplify pronunciation. "Can't" just reflects the pronunciation. However, it is considered incorrect to write contractions like this in formal writing. You would never seen an academic paper, for example, using "can't," "don't," "wouldn't," etc.
As to the Japanese question, there are definitely linguistic rules, but it's hard to define them—the rule actually is incomplete. I wrote a computer program about 5 years ago that generated proper Japanese based on the rule, but the rule is not always right. I can't find the program anymore, but I based it off a wikipedia page (linked below, I think).
Suffice to say, with about a year to two years you will have the rule internalized completely unintentionally, and very rarely will you be surprised by any exceptions and only sporadically will you get it wrong. And, just so you know, the voicing/devoicing you're speaking of is called "rendaku," 連濁.
Rendaku - Wikipedia, the free encyclopedia