Attachment is defined as a mental affliction, which is a habit that robs causes us suffering. Attachment arises like this:

We encounter something pleasant + inappropriate attention/exaggerating its good qualities = attachment arises (the feeling we can not be happy without it)

Sometimes we are attached to people being different than they are, to being right, drugs, alcohol, or unhealthy food. In all these cases, it is not the person or object that makes us unhappy, but our attachment that causes us to suffer.

What attachment is not:

Letting go of attachment does not mean that we don't own things, but that those things don't own us.

Letting go of attachment doesn't mean that we don't have love others, but that we love them without attachment.

Longing gives rise to grief;

Longing gives rise to fear.

For someone released from longing

There is no grief;

And from where would come fear? (212)

Affection gives rise to grief;

Affection gives rise to fear.

For someone released from affection

There is no grief;

And from where would come fear? (213)

Infatuation gives rise to grief;

Infatuation gives rise to fear.

For someone released from infatuation

There is no grief; And from where would come fear? (214)

Sensual craving gives rise to grief;

Sensual craving gives rise to fear.

For someone released from sensual craving There is no grief;

And from where would come fear? (215)

Craving gives rise to grief;

Craving gives rise to fear.

For someone released from craving

There is no grief;

And from where would come fear?

—Buddha, The Dhammapada

References

Buddha. The Dhammapada, translated by Gil Fronsdale. (2011). Shambala, pp. 56-57