What makes the lamb love Mary so?

Mary had a little lamb,
Its fleece was white as snow;
And everywhere that Mary went,
The lamb was sure to go.

He followed her to school one day,
Which was against the rule;
It made the children laugh and play
To see a lamb at school.

And so the teacher turned him out,
But still he lingered near,
And waited patiently about
Till Mary did appear.

"What makes the lamb love Mary so?"
The eager children cried.
"Oh, Mary loves the lamb, you know,"
The teacher then replied.