After looking at the synastry charts between me and some others I have met in life, I'm starting to think this is how synastry works:The contact made with the planet is the thing that brings forth the energy of the planet; while the aspect on the planet is what brings about the conclusion...or destiny of that energy. And the tightness of the aspect shows how soon or how often said energy will come about in the relationship.
For example, If you have a synastry with someone that has many squares to your mars, you will feel sexual attraction with that person... but if that sexual relationship were to play out there will eventually be sexual revulsion - like WHAT DID I EVER SEE IN YOU? Where the thought of the person touching you might actually make you feel repelled. If you have a synastry with someone that has many trines to your mars you will also feel sexual attraction, but if a sexual relationship were to flourish it will remain positive and both parties will continue to enjoy that part of the relationship. In both cases mars is being contacted a lot. But the square aspect has a negative conclusion/destiny, while the trine aspect has a positive one. And then there is of coarse the tightness of the aspects. I find that the tighter the aspect the sooner the energy becomes noticeable in the relationship. If two people have great aspects in really tight orbs and some really negative aspects in wide orbs, the positive nature of the relationship will last longer as the negative changes won't show until later (and not as often).
Venus: If you have synastry with someone who makes many trines to your venus, the flow of love between you will have a more positive conclusion. You may fall in love and also receive love in return (even if the other person doesn't actually fall for you in that way). If you have synastry with someone who makes many squares to your venus you may also fall in love... but you will likely end up feeling heart-broken and rejected.
I realize that this theory is only really testable if you have people who make MANY aspects to a particular planet because just one or two aspects to a planet might make it hard to distinguish, but I still thought I'd put it out there.