In short, habits and conditioning. You obviously can't get an educated read on your opponent without getting information first. Information is key.
Here's an example, as Ganon, when I flame choke (side-b) I don't always down-tilt after because I want to know what my opponent is going to do. Does he like to get up attack? Roll away or behind me? I'll try it maybe twice and react accordingly. Looks like my opponent likes to roll behind me because maybe they think I'm gonna down-tilt or flame choke forwards again. Now's your chance! Let's say this is a character you cannot flame choke into down tilt. He's at 120% and you flame choke. You now can make an educated read and say this person is gonna try to roll behind me. Jump back and hit em with a down-air. Stylish read and a KO.
Here's one more example. Your opponent is on the ledge and you're let's say Roy. You spam down-tilt while they're invincible and they get up attack. They're like "Lol you got hit by my get up attack". You get them off stage again and they're on the ledge again. Do 1 down-tilt before they can get up attack and then shield. They might get up attack because you fell for it last time. Well... not this time. Shield it and you forward smash them for the KO (I'm assuming it's fast enough to connect).
You cannot simply just read your opponent without playing them a little bit. It can take as little as one stock to know what kind of player they are, possibly sooner. A lot of the time if I see someone get up attack twice that's all the info I need to know to make them eat it every time.
This can vary depending on the skill of the player. At higher levels people may tend to change up their style and options so you must also be wary of that.