You must have seen that a manglik always marries a manglik and parents doing kundali matching at the time of marriage.
In Vedic astrology, Mars, Saturn, Rahu, Ketu and Sun are considered cruel planets. Mars is the cruelest planet afflicting the birth horoscope of a person. If placed unfavorably, Mars is the most damaging planet for the married life.
Mars positioned in first, second, fourth, seventh, eighth or twelfth house in a Kundali creates Mangal Dosha. Mangal Dosha should be checked not only in Lagna chart but also in Moon chart and Venus chart. A person is Non-Manglik only if none of the three charts is afflicted by Mars. Mangal Dosha is also known as Kuja Dosha and Bhauma Dosha. The person having Mangal Dosha is known as Manglik.
Mangliks have too much energy and it can be compared to a raging fire. Fire, if used well, can help create something new and therefore, this energy needs to be channeled correctly. They need to be constructive or they will end up destroying themselves. Mangal Dosh affects individuals in many ways - it affects their married life - sprouting troubles, causing distance and difference between the couple. It also affects one's mental state and leads to losses in finances. However, if a person is born with Mangal Dosh but born on a Friday, the effects are nullified. If two Manglik individuals get married, the malefic effects are cancelled.
When Mars is positioned in the 1st house: 1st house represent life partner or spouse. Hence, with Mars in the 1st house, it will hamper the married life. It can lead to physical assaults and domestic violence. The couple will suffer from tensions, unhappiness, distress and it can end in divorce. When placed in 2nd house, it affects the family life if the individual. Unforeseen troubles, delays and misunderstanding are common. It also leads to underperforming in their professional lives. With Mars placed in 4th House, it will affect the person's career. They might have to switch jobs and face problems professionally. Financial troubles will follow soon. If Mars is in the 7th house, such an individual will suffer from aggression and behave in an ill-tempered way. They will try to be dominant in all their relationship and dictate. They may also cheat. When Mars is placed in the 8th house, the person will become a sloth - super lazy and ruin their reputation with their elders as well as at work. They will lose out on paternal property. With Mars in the 12th house, it gives rise to enmity, mental troubles and losses.