What causes vaginismus?

Vaginismus is considered a disorder of sexual dysfunction. It has several possible causes, including past sexual trauma or abuse, psychological factors, or a history of discomfort with sexual intercourse. Sometimes no cause can be found.

Sharon has given you a good answer, you may like to look at the link below for some more detailed info.

Vaginismus is when the muscles around the vagina tighten (contract) against your will, causing the vagina to spasm. In some cases, this can be quite painful.
It is a psychological problem that shows itself in a physical way and is fairly common among women, especially women in their late teens to thirties.
The vagina muscles go into spasm, usually in response the vagina or vulva being touched before sexual intercourse. It can also happen if penetration of the vagina by the penis (sexual intercourse) is attempted, or during a gynaecological examination. Some women find that they cannot use tampons when they have a period (menstruate) because of vaginismus.
Vaginismus can cause distress and relationship problems. It can also create problems when it
http://www.nhsdirect.nhs.uk/articles/art...

Here are some more sites that might help you understand this symptom:

http://www.nlm.nih.gov/medlineplus/ency/...
http://www.psychnet-uk.com/dsm_iv/vagini...
http://www.sexwithoutpain.com/vaginismus...
http://www.umm.edu/ency/article/001487.h...
Treatment
http://familydoctor.org/online/famdocen/...

What are the causes of vaginismus the vaginistic reaction?

Answer:
There are many possible causes of vaginismus. They are briefly summarized below:
Physical factors:

Pain in the vagina, which leads to fears of this pain at the next love play and to the woman tightening her pelvic floor muscles.
Psychological factors:
Fear of pain, the penis, pregnancy or an aversion to the partner.
Social factors:
Certain norms about sexuality in the education.
Religious taboos.
Traumatic experiences like a painful deflowering, assault and rape, sexual abuse or other harmful events.
Relational factors:
Factors in the relationship like resentment and negative feelings about the partner, sexually and in a broader sense.
Clumsiness of the partner.
Insufficient foreplay and insufficient arousal.
Personal difficulties that become evident in the relationship.
Traditional ideas about sex.
All these factors lead to thoughts or feelings that make the pelvic floor muscles tighten when the penis or tampons come near.

This is directly from the website I've listed as my source:
Vaginismus is involuntary - not intentional
It is important to note that vaginismus is not triggered deliberately or intentionally by women. It happens involuntarily without their intentional control and often without any awareness on their part. Vaginismus has a variety of causes, often in response to a combination of physical or emotional factors. The mystery of the problem can be very frustrating and distressing for both women and their partners. Despite the fact that vaginismus is involuntary and can strike any woman, many women feel intense shame from being unable to have intercourse and keep their pain private, feeling uncomfortable sharing their secret with anyone.