[groby_b]

Very simple explanation: Entries in double-entry accounting always need to balance - you need two.

So if somebody gives you $100 in cash, you book that as $100 in your "cash" asset. But... the books must balance! And so, you need to book -$100 on the "income" side, because that's where the cash comes from.

If you're not happy with "because it needs to follow the rules" alone, replace "income" with "other people's money". For you to get cash, somebody else needs to give up cash.

The reason you differentiate between asset/liability and income/expense accounts is that you can't really fully balance the latter two. You don't know all the details happening to "other people's money". You know everything happening to your assets and liabilities.

As a result, income/expense are accounted over time ("I made $x/year") while assets liabilities are a value ("I have $357 in my cash drawer") - for the former two, all you know is the delta over time you cause.