Well as you can see from everything written above me I didn't come here to write a plot synopsis or even to give much discussion of the film.
Perhaps the most significant (and oft repeated) part in the film for me is the following:
You're waiting for a train.
A train that will take you far away.
You know where you hope this train will take you,
but you can't know for sure.
How can it not matter to you where this train will take you?
Because we'll be together.
Two things of fundamental importance jump out at me from this dialogue: Faith and Love.
Now, although I initially approached this from a religious--though I prefer to think of it as existential--perspective, I won't bore you with the details of that. What is important is I stress by faith I do not mean belief (in God). I simply mean a confidence, trust, assurance, devotion, loyalty, or commitment. The crucial point that this back and forth between Mal and Dom emphasised for me is our dependence on these harmonious opposites-faith and love--to give us rhyme and reason in our lives.
From the film we can see that Mal lost her faith in reality. Not only that but her faith in everyone she cared for. Her husband and children (as you'll notice I chose to accept the more optimistic ending). Mal lost her faith in these things but kept her love for them. With only love but no bond she was essentially turned into the villain of the movie. She was scary at times--giving the film it's thriller edge. She became a monster in her love for things (seeking them was her only intent)and without maintaining a real commitment or bond to them.
Perhaps this says something about the human condition. With only love we can become obsessed. Infatuated, fixated, consumed, preoccupied, and fanatical. We become dangerous in one way or another. In the film, Mal was selfishly focussed on pulling Dom towards her. It would have broken all ties with reality. With no emotional or social ties they would have been dehumanised. Essentially we need to hold both faith and love in equal measure to maintain our humanity.
I've discussed faith without love but what of love without faith. Well love already heavily implies that very faith. The commitment to another. If you need a definition think 'Do unto others...', Abraham's sacrifice to God, Christ's sacrifice to Mankind or John 3:16. If you're diametrically opposed to bible references, think of love as doing something for someone else even though you know it'll make you unhappy. Breaking up with your dream girl because you know she'll be happier with someone else.
So if you don't hold that love towards another person. What's left? You substitute the role of the human. Greed, consumerism and materialist sentiments spring to mind. it leads to a single person accumulating more wealth than they could spend in several lifetimes. Take bankers for instance. Demanding ever more wealth even after having achieved this. Perhaps in reaction, resentment, reparation for the realisation of futility of your own mortality. The kind of sentiment that argues rather lamely: 'whoever dies with the most stuff wins'.
Anyway, back to what I was on about. So we require that coexistence of faith and love. Where do we find that exactly? Well I mentioned before that people may choose to see it in commitment to God. Since you can't know for certain if there exists a deist God (just as neither of our main characters know where the train will take them) faith in God becomes becomes not belief in the supernatural but a bond between men.
However as I said, it doesn't have to come from an existential or religious premise. It may come from anywhere that is strong and resilient enough for you. For most people, even those who are religious, it will come through family. Another thing the film emphasises is the importance of family. Nonetheless, wherever you choose to seek the foundation for this 'faith' it helps if it's something deep and impenetrable like the ontological concern for the nature of God that gives you a solid basis for living in faith and love with others.
So here it is. This wasn't meant to be taken as a religious homily. It's simply my way of emphasising how this film has an overwhelmingly optimistic (or, Christian) theme. At least for me. But maybe you think I'm looking too hard. Seeing what I want to see.