Although the speaker meets with his neighbor each spring to mend the wall, he thinks that the wall is unnecessary. Why?

The speaker thinks that the wall is unnecessary because the border between the two properties is already obvious, and because there are no animals to be fenced in by the wall. In other words, the wall serves no logical purpose either as a boundary or as a corral:

There where it is we do not need the wall:

He is all pine and I am apple orchard.

My apple trees will never get across

And eat the cones under his pines, I tell him.

He only says, "Good fences make good neighbours."

Spring is the mischief in me, and I wonder

If I could put a notion in his head:

"Why do they make good neighbours? Isn't it

Where there are cows? But here there are no cows.

As you can see, the speaker explains that his own property has only apple trees, while his neighbor's property has only pine trees. Anyone can tell the difference between the two, so it's already clear where the neighbor's land ends and the speaker's begins. The wall, then, is unnecessary. Why bother having it, especially if you have to repair it every year? Also, it would make sense to have the wall there if each neighbor had his own cows, or even if only one neighbor had cows. The wall could physically keep the animals confined to their proper field. But there are no cows!

Since it's clear now that the wall serves no immediate function we'd expect it to, the natural follow-up question here is, "Since it does nothing, why do the neighbors keep the wall?" or "Why is this useless wall the subject of, title of, and chief symbol of a famous poem?" The answer is probably that the neighbors bond together over the wall repairs, year after year. It's funny, because while walls typically keep things hidden and separate, in this case the wall brings the two neighbors together.