SCUBA QUIZ: How Much Weight Do You Need?

The right amount of lead on your weight-belt is important to dive comfortably. You do not want to sink like a rock and use your air to compensate this. Struggling to go down or holding on to a rock during your safety stop might even be worse.

Your body weight/size is one of the main things. Fat is more buoyant (float) than muscle mass so smaller people need less weight in general.
The kind of wetsuit you are using. Wetsuits are often made from neoprene which is buoyant. The more neoprene, the thicker and longer the wetsuit, the more buoyant you will be.
The kind of BCD and fins you are using. There can be huge differences in how much fins and a BCD can weight and their buoyancy. Nowadays you can travel light with a special travel BCD and travel fins, but you might need more weights to compensate in the water... (Thanks for the add Tom!)
How salt is the water? Salt gives you buoyancy. There can be a huge difference between a fresh water (lake) and a salt water (ocean) dive. Oceans have differences in salinity as well, the Red Sea is much salter than the Gulf of Thailand for example.
What kind of tank are you using? Are you using one or two tanks? Aluminum tanks are more buoyant than steel tanks.

Surface Marker Buoy Funny Dolphin

Your experience level. The better you dive and move like a fish, the less air you consume and the less weight compensation you need. Newly certified divers often drop a few kilo's within the first 10-15 dives, depending on how much time is in between the dives.
Dive tools and accessories. Divers knife, SMB, torches & accupacks and underwater cameras can influence your buoyancy, especially if you bring a lot. A small GoPro will not make a big difference but think about the huge cameras dive professionals are bringing. The housing can be buoyant because of the air trapped inside, while heavy external strobes/lights can make it sink.