Courts want to hear how well or bad the child is doing in school; how the child interacts with the other parent; who has the primary parent been in the past; are there alcohol or drug problems with either of the parents (must have evidence of this- allegations alone will not hold up); is the child going to have to move schools; is the child in therapy- if so, why and how are they doing; does the child have any disabilities or special needs; how well do parents communicate with each other and are the parents bad-mouthing. These are just a few things that can make up best interest.
I have seen too many parents get on the stand and state what is better for them, what is convenient for them and how horrible the other parent is. Courts do not want to hear this, you can often piss off the judge with this type of testimony. Now, that doesn’t mean you can’t state your concerns, you absolutely need to do that but keep the emotional hatred out of it.
The long and short of it is that “best interest of the child” is very arbitrary and subjective.