Why is it hard to scale a database, in layman’s terms?

I generally don’t like to link to things that require that you sign in to access the content, but I thought this question and answer on Quora was worth sharing. The question is: “Why is it hard to scale a database, in layman’s terms?” To which Paul King put forth a useful and easy to understand answer.

King identifies the four main problems with scaling databases:

Search
Concurrency
Consistency
Speed

King doesn’t put forth answers to these questions (because the answers vary), but an understanding of the core problems is the first step on the road towards solving them. If you have a Quora account (which is free), take a couple of quick minutes to read this.