Tied in knots
People who find it erotic to be tied up find it so for a variety of reasons
The most frequently cited reason is a mental freedom from inhibitions and responsibility since they have, in a way, given up control of the sexual situation to follow. This is sometimes referred to as a "power exchange."
Some like the tactile feeling of restraint, that is, the feeling of pressure or pulling. Some enjoy the feeling of helplessness for its own sake.
Some like to struggle aggressively against their bonds, particularly when being sexually or otherwise stimulated.
There are some in this category who play bondage games that do not include a significant sexual component.
To intensify the experience of orgasm control or of orgasm denial.
Some derive pleasure from symbolic degradation (less common). People who enjoy role-playing prison or mental hospital situations probably fit best in this category.
Fetishistic interest in the mechanics of bondage, with particular interest in the equipment and restraints used. Some of these people are interested in the look, feel, and aroma of leather and rubber restraints. Others are fascinated by the relationship between the geometry of the tie, the degrees of freedom remaining and the feelings elicited.
As an adjunct to other BDSM activities
Like hang gliding or mountaineering, some feel that bondage allows them to do something potentially dangerous in a safe way.
In the wide range of human sexual experience, there are probably a few others.
People who enjoy tying other people up are motivated by a variety of reasons, including:
Taking pleasure in the erotic submission of their partner
The feeling of trust, which comes from another person placing their physical freedom in their hands
Wishing to please their partner, and the stimulation engendered by their partner's pleasure in it
Fetishistic interest in the elegance of bondage, with particular interest in the geometric patterns and symmetry (or artistic asymmetry) of the restraint
Using bondage as an adjunct to other BDSM activities
Enjoyment of the power and control one has over a restrained partner; people for whom this is a principal motivation may have trouble making it much fun for the other person
resource information-http://en.wikipedia.org