My goodness you have a lot of questions...

[Sidebar: you might try posting one question at a time; you're likely to get more responses.]

I think we were meant to have the capacity to feel pain, because pain is a teacher.

As far as crying goes, I'm a huge fan. I often cry at seeing / reading / hearing something that is beautiful.

I think that crying is good for our inner emotional selves: it's a way of expressing very deep (and often painful) feelings.

-ITL-

Seinfeld: "What is this salty discharge coming from my eyes?!"

Crying is an outlet, especially for things out of your control, like death of friend or a loved one.

Yes we were meant to feel pain, so that we can contrast it with happiness.

Erhm...well, we (at least I) do not cry only because of pain, but also because of joy. Nothing brought me more to tears then the song Con te partiro (time to say goodbye - and I consider the English title to be wrong) by Andrea Bocelli, which for me it's all but sad; or some Disney films; and I am not talking about sad scenes. I see crying as an arousal of emotions: being it woe or being it mirth. So, I'd say it does not predispose pain.

I know not if we bulit this ability: since it was granted we can make use of it.

Were we meant for pain? I will just quote Blake on the matter:
"Man was made for joy and woe;
And when this we rightly know
Through the world we safely go.
Joy and woe are woven fine,
A clothing for the soul divine.
Under every grief and pine
Runs a joy with silken twine."
- Auguries of Innocence

I hope I was of help,

Debate to why exactly we cry is still ongoing with various theories being proposed. There seem to be some evidence that seem's to point to crying being a effective at lowering levels of stress in individuals which has been put forward, as well as several other theories which could also be valid. Crying is perfectly normal and probably good for us once in a while, I know a good cry can make feel better on occasion.

I believe that crying is a neccessary mechanism for expressing heavy emotion, be it pain, love, fear, or beauty.

It's a natural emotional-physical response.

-ITL-

Sadness if well expressed is beautiful.
The Blues illustrative this best in music, Miles Davis espec My Funny Valentine.

Maybe that isn't 'feel my sadness' but surely appreciate it instead?

What examples of both sadness and joy have triggered this catharsis in you?

Would you share your tears be they pain or joy? is either one different to its expression and reason for sharing even hiding?

Would you rather not share what gives you pain as opposed to happiness?

Are tears the climax of both pain and joy, and because they have the same outcome are not in some nature the same?
Something about orgasms being a form of pain.