What is Mangalik Dosha?
When mars is located in any of the houses shown in the horoscope, the horoscope is said to have "Mangalik Dosha". i.e. if mars is in first house, fourth, seventh, eighth or twelfth house person is having "Mangalik Dosha".
Why is it considered as a drawback during marriage?
mars affects the 4th, 7th & 8th house from the house in which it is placed. This is called "Mangal Drushti".
mars is a planet which is representative of fire, electricity, chemicals, weapons, aggression, high energy, blood, fight, accident etc... Let's see what are the repercussions of mars located in these houses. Whichever house it is located or has its drushti, effects of Mangal are experienced in the aspects of life represented by that house.
1) 1st house: if mars is placed in the 1st house it will affect 4th, 7th & 8th house in addition to 1st. 1st house represents the personality of the person, hence person may be very short tempered. 4th house represents the house, vehicles of the person, hence there is a possibility of problems associated with house & vehicle e.g. accident due to fire, chemicals, electricity etc...7th house represents the Marital Life, spouse & business in partnership, hence there is a possibility of turbulent married life, spouse may be of very hot tempered nature, loss in partnership etc... 8th house represents death, sudden monetary gains etc... hence there is a possibility of fatal accident to the person. Of course these are very broad guidelines. Many other angles need to be studied like the overall quality of the horoscope, power of the planets, aspects of the planets etc...
2) 4th House: If it is placed in the 4th house it will affect 7th, 10th & 11th house in addition to 4th house. We have seen effects on 4th & 7th house. 10th house represents career, father, sleep etc... hence there is a possibility of frequent changes/disturbances in the career, sleep disorder, issues with father or even early death of father etc... 11th house indicates the monetary gains in life, hence there is a possibility of losses due to accidents, theft etc...
3) 7th House: mars in 7th house affects 10th, 1st & 2nd house in addition to 7th. 2nd house is the house that gives the idea about the wealth of the person. It indicates the family of the person & it is also the 8th house from 7th house indicating death of the married life/business in partnership. Hence mars drushti on this house can create issues among the family members, there is a less harmony among the members due to short tempered & aggressive behaviors. So also there are chances of loss of money.