There is a quote I once heard “Good teams are usually great for the same reasons, bad teams are almost always bad for different reasons”. This is the much the same for people. Generally speaking, the best people you have worked with are very similar in terms of what they provided that made you really happy to be with them. Example “good” traits:

- honestly - dependability - integrity - intelligence - humility - excellence - generosity - proactivity

Toxicity happens when people exhibit the opposite of any of those. A favorite example is folks who constantly bad mouth other people on the team. Well, if you work with that person, you just know they are complaining about you behind your back as well.