Communication
Many a relationship has been screwed due to a lack of communication. You should know your partners wants, needs, hopes, dreams....

Trust
Your basically trusting your partner with your most precious and most fragile item. Your heart. you should feel safe doing so.

Honesty
Neither of the first two would be possible without full disclosure between both parties.

Respect
Each person needs to respect their partners differences and realize they shouldn't try to change them, they fell for them as they are.