Each planet (Graha), except for the Sun and the Moon, own two signs (Rashis). When the planet is placed in the sign it owns it becomes strong and behaves in the most natural way. It is like a person acting freely and most naturally when he/she is at home, so are the planets behave most themselves and with strength when they are in one of their own signs.
Sun owns Leo
Moon own Cancer
Mercury owns Gemini and Virgo
Venus owns Taurus and Libra
Mars owns Taurus and Scorpio
Jupiter owns Sagittarius and Pisces
Saturn owns Capricorn and Aquarius