Personally, I believe the Nine of Hearts presents a huge challenge: allowing yourself to be filled with positive emotions. At it's best, this card stands for humanitarianism, the need to serve the others, the ability to love and feel compassion without letting your personal judgements interfere. And no, this isn't easy – none of the Nines are easy.
In a more "down to earth" level, it could mean not letting your feelings suffocate you or turn into poison inside you. This can be achieved by cultivating altruism and other noble emotions.
At it's worst... it's very bad. Extreme emotions and mood changes, obsessions, feelings that "drown" you, paranoia, inability to control your impulses, distress, decadence, excessiveness... Like all in Nines, it may mean surrendering yourself completely to one "element" only (water/emotions, in this case), and forgetting all others. The necessary balance is lost.
Of course, the "meaning" of the card will be determined by the whole spread, the position in which it appears, the question asked etc. There's not need to be afraid whenever this card shows up - it looks ugly, but it has a beautiful potential. Try to figure out where the excess is, how it may be hindering you (or your sitter) and what can be done to restore balance.
