In Hindu mythology, Ketu is generally referred to as a "shadow" planet. It is believed to have a tremendous impact on human lives and also the whole creation. In some special circumstances it helps someone achieve the zenith of fame. Ketu is often depicted with a gem or star on his head signifying a mystery light.
Ketu is the planet denoting detachment from worldly desires. While its head component Rahu is avaricious and a greedy and scheming go-getter(the degree is determined by what sign Rahu is placed in), Ketu is the planet representing Connection with the Divine, supernatural and meta-physical phenomena.
Ketu is most uncomfortable in the Airy signs because it is headless with no intellectual acumen and the added responsibility of "thinking", "intellectualising and analysing"(typical traits of the Air element) makes it feel chaotic and squirmy.
It feels most comfortable in the signs of Cancer, Scorpio and Pisces. These are watery signs and indicate mutability, emotions and in the division stand for WATER that belongs to no specific LOK but are transcendental.
4,8,12 or the 23 watery signs are the MOKSH signs(as per the Hindu partition of Dharma, Artha, Kama and Moksha as the 4 pillars of human life purposes) and having the divine Ketu in any of these signs especially Scorpio and Pisces makes a person very fruitful in its functions denoted by Ketu.
Ketu wants to conect with the Universal energy and aims towards attaining Moksh or the universal NOTHINGNESS which Hindu religion talks about.
Ketu is happiest when IT IS. It wants neither happiness, nor sadness, neither comforts nor penury. It just wants to unfathom the mysteries of life by BEING. And the perceptive, occult oriented, deep, penetrating sign of Scorpio and Pisces are where it thrives.
And Ketu by Parashari astrology school is regarded as exalted in SCORPIO.