Comments like that might be annoying, but they're not wrong.
Your posture, the way you hold your body when you're sitting or standing, is the foundation for every movement your body makes, and can determine how well your body adapts to the stresses on it.
These stresses can be things like carrying weight, or sitting in an awkward position.
And the big one we all experience all day every day: gravity.
If your posture isn't optimal, your muscles have to work harder to keep you upright and balanced.
Some muscles will become tight and inflexible, others will be inhibited.
Over time, these dysfunctional adaptations impair your body's ability to deal with the forces on it.
Poor posture inflicts extra wear and tear on your joints and ligaments, increases the likelihood of accidents, and makes some organs, like your lungs, less efficient.
Researchers have linked poor posture to scoliosis, tension headaches, and back pain, though it isn't the exclusive cause of any of them.
Posture can even influence your emotional state and your sensitivity to pain.
So there are a lot of reasons to aim for good posture.
But it's getting harder these days.
Sitting in an awkward position for a long time can promote poor posture, and so can using computers or mobile devices, which encourage you to look downward.
Many studies suggest that, on average, posture is getting worse.
So what does good posture look like?
When you look at the spine from the front or the back, all 33 vertebrae should appear stacked in a straight line.
From the side, the spine should have three curves: one at your neck, one at your shoulders, and one at the small of your back.
You aren't born with this s-shaped spine.
Babies' spines just have one curve like a "c."
The other curves usually develop by 12-18 months as the muscles strengthen.
These curves help us stay upright and absorb some of the stress from activities like walking and jumping.
If they are aligned properly, when you're standing up, you should be able to draw a straight line from a point just in front of your shoulders, to behind your hip, to the front of your knee, to a few inches in front of your ankle.
This keeps your center of gravity directly over your base of support, which allows you to move efficiently with the least amount of fatigue and muscle strain.
If you're sitting, your neck should be vertical, not tilted forward.
Your shoulders should be relaxed with your arms close to your trunk.
Your knees should be at a right angle with your feet flat on the floor.
But what if your posture isn't that great?
Try redesigning your environment.
Adjust your screen so it's at or slightly below eye level.
Make sure all parts of your body, like your elbows and wrists, are supported, using ergonomic aids if you need to.
Try sleeping on your side with your neck supported and with a pillow between your legs.
Wear shoes with low heels and good arch support, and use a headset for phone calls.
It's also not enough to just have good posture.
Keeping your muscles and joints moving is extremely important.
In fact, being stationary for long periods with good posture can be worse than regular movement with bad posture.
When you do move, move smartly.
Keep anything you're carrying close to your body.
Backpacks should be in contact with your back carried symmetrically.
If you sit a lot, get up and move around on occasion, and be sure to exercise.
Using your muscles will keep them strong enough to support you effectively, on top of all the other benefits to your joints, bones, brain and heart.
And if you're really worried, check with a physical therapist, because yes, you really should stand up straight.