The answer to the question in the title of this post is almost always very evident as soon as one sees the patient. He or she is sick and needs help to cope with the sickness or illness and get on with life.
There could be other reasons as well: fear of suffering, seeking relief in a manner better than what is being offered now (second opinion), seeking reassurance and so on. Almost always there is a child like need that has to be addressed by the doctor when seeing a patient.
In being a patient, it is assumed by the doctor that the patient is not there to increase her suffering. It is also inconceivable that the patient has any malicious intent in presenting to the doctor.
An exception to such a generalization is any work-related health issue: here there is some psychological game playing going on,sometimes subconsciously. I have rarely seen any malicious intent in a domestic setting.
Sometime back a lady in her mid forties started consulting me