There's lots challenging parts to this job, but few are as variable and rapidly changing as managing the people in your lab. When everyone is working well together it is great, but the pendulum always swings the other way at some point. Each new member of the lab changes the dynamic in some way. It doesn't have to be positive or negative, but there is always some shift. The trick is making sure things don't go too far bad when they do start to slip off the rails.
Everyone has baggage. And that includes you. We are all, to one degree or another, a product of the management we have experienced. Perhaps you've been through a few labs and have an idea of how you want to approach your mentoring, but the range of possible is far greater than any one person is going to be subjected to. Trainees coming from another lab or making your lab their first stop, similarly have expectations that are either learned or imagined. The first few months is important in terms of setting expectations for both sides. Your availability, their hours, communication, dealing with data, lab protocols and SOPs, etc., etc. Sometimes you'll need to real people in and other times you'll need to force them to take more line.
What's nearly impossible to pre-manage, IME, are interactions among your group. This is largely because much of it is going to be hidden from you unless things get pretty far down the road. You'll always be playing catch-up and getting partial information. Many of us are competitive by nature, and that can manifest in both legitimate and ridiculous ways in a lab. Much of it doesn't require intervention, but occasionally it is necessary to get people together and figure out where the problem is and what can be done. If you're lucky you catch it early and can find a way to ensure a working relationship between the parties. If you're not, well, that's less fun.
What I find helps is having a fairly open policy about drop-ins and conversations with anyone in the lab. It's not always convenient or helpful for my productivity, but I can't really afford to have the productivity in the lab go south over personal frictions, so I make the time. For me, open communication with the people in my lab has been critical to making sure there is open communication between them. Usually I can see the seeds of developing conflict if people feel comfortable talking to me about what is and isn't working for them.
Have you been in a lab where everyone got along great? Have you been in an unhappy lab? Have you seen labs shift from one to the other? Was the cause a new person or people, or did it relate to a shift in management style?