One of the biggest reason not to compare your child to others is that it hits their self-esteem hard. How would you like to be constantly compared to others? Low self-esteem issues usually have their root in childhood, so don't make your child feel that they're inferior to others. You don't want them getting conceited, but nor do you want them feeling worthless.
Every Child has Own Interests and Abilities
Each child is an individual, and has their own talents and interests. Why does it matter if one child is musically inclined and another isn't? The non-musical child may be good at sports or loves reading. One thing is not better than another, and neither is one child better than another. Let them develop their own interests and abilities. They should not feel that they have to be more like a sibling or your friend's child.
Direct comparisons can make a child feel very resentful. When it's made clear to them that they're not as good as a sibling at something, they not only feel bitter towards you, but also towards the sibling. Do you really want to foster antagonism between your children? No you do not. So avoid comparisons; even if you think they won't find out, someone may tell them.
Comparison: Sticks around and Stings Later
You may be surprised at how long the memories of comparisons can stay with a child. Long after you've forgotten saying 'why can't you be more like your brother?' your child will remember being made to feel that they weren't good enough for you. The message soon sinks in, even if you didn't mean for them to feel that way; it hangs around for a long time.
It also risks creating a self-fulfilling prophecy. Over time your child will absorb the message that they're not good enough, and start to believe it. Why bother trying if they're not going to succeed? And so that's precisely what happens. The child is effectively set up to fail.
Childhood isn't a race to the top; children develop at their own pace. Yet parents often worry because their child isn't walking at the same age that their friend's child did, or has a much smaller vocabulary. There isn't a set timetable for a child's developments though, so don’t worry about when your child reaches a milestone. They'll do things in their own good time.
Superiority Complex: Big Headed
Conversely, it's also wise not to compare your child too positively to others. Do you want your child to grow up thinking that they are superior? Praise is good, but to some extent. Constantly telling your children that they are so much better than others; you're not doing them any favours; you raise them to be big-headed.
Comparing your child to others is likely to give them a negative image of themselves.
Parenthood isn't a competition, and you don't have to show other parents how much better your child is, or worry that other children are doing better than your own. Gently encourage them to believe in themselves, and help them discover their own abilities.
Have you ever been compared to a sibling, and how did it make you feel?