Why Moon and Mercury are rivals

1.    Reason being, Moon is all about emotions, motherly care and Mercury is about calculations, logic and rational thinking. When they both collide with conjuction or 7^th house aspect, there is serious mismatch in signification they represent. If Moon is strong, then natives logical and rational reasoning is dependent on the emotions and feelings which is not good for worldly transactions. If Mercury is strong, emotions / feelings are driven by logical thinking / calculations which is not good for personal life. If both are strong have seen in many cases there will be a break up after a long love relationship (Mercury about love life and Moon emotions). It could also indicate a love failure.
     Indian Mythology on these 2 planets - Mercury is believed to be illegitimate son of Moon. Hence Mercury hates his father for being tricked his mother, but Moon is neutral towards his son.

s   For the same reason, Mercury placed in Cancer is neutral. Moon placed in Gemini / Virgo is not a healthy placement for Moon as Mercury being lord of these 2 houses renders Moon weak. Support for Moon in this case can come from aspect of functional benefic Jupiter or aspect of Sun (only 7th aspect). Aspect of Sun gives him strength as it implies a Full Moon.
     Let us analyse the chart mentioned above.
     Mercury - 2nd house - Virgo 
     Moon - 8th house - Pieces
     Mercury is stronger in this case as he owns moolatrikona sign. Moon is strong too         occupying a water sign but rendered weak on his placement in 8th house. Moon can get support from well placed Jupiter - lord of Pieces or an aspect from Jupiter. Sun, Mars and Jupiter are functional benefics for Leo ascendent and hence an aspect of anyone will support Moon.

      Jupiter in Sagittarius – Lord of Moon in moolatrikona and in 5^th house.

       Jupiter in Pieces – Lord of Moon in own house and in conjuction

      Jupiter in Cancer - Lord of Moon in exaltation and having 9^th aspect.

      Jupiter in Virgo – Lord of Moon having 7^th aspect.

      Jupiter in Scorpio - Lord of Moon in friendly house and having 5^th aspect.

      Jupiter in Aries - Lord of Moon in friendly house and in 9^th house.

      Jupiter in Leo - Lord of Moon in friendly house and in 1^st house.