This is an excerpt from this webpage.
They're [novice players] also dangerous and can put you off your game. How many times have you been on Party Poker playing $1-2 No-Limit Hold 'em and raised 4x the big blind with A-K and get called by A-5 only to watch the flop come A-6-5 rainbow. "Great!", you're thinking as you reraise your clueless opponent for all his chips. You don't catch a K on the turn or river and you're left wondering why he stayed in the hand at all.
So, the flop is A-6-5 rainbow. It's implied that going all in with A-K is a good idea, but calling that with A-5 is a bad idea. Why? I mean, if we look at this from the position of the calling player:
I have an ace high two pair, so the only way the other player could already have a superior hand is if they had A-A or A-6, both of which are fairly unlikely. They can't catch a flush since the flop is rainbow, and even if they have exactly the right cards, like 4-3, they're still unlikely to get a straight. So my two pair is fairly likely to come out on top.
Now I guess you could argue that "fairly likely" doesn't justify going all in, but then why is it okay for the first player, who had A-K, to go all in? They have even worse odds.