If you haven't seen the film and don't want your future viewing experience spoiled, read no further. Go water your plants or something.
In the film, through some freak accident committed by Father Time, the characters that Sandra and Keanu play fall in love. Never mind that they've never seen each other, never mind that they communicate only via a magic mailbox, never mind that Sandra's character's now is two years ahead of that of Keanu's character. They have a unique long-distance (in a weird sort of way) love affair, they cheat death, they live happily-in-the-now (finally), yadda yadda.
What I'm getting to is this: Sandra tells her mother of her extraordinary love affair while Keanu confides in his brother. The movie implies that these relatives accept these strange tales told by the protagonists (which is saying a lot for them. Frankly, I'd think of calling the men in the white coats if a relative of mine tells me a story of having an inter-dimensional love affair). What the movie doesn't specifically show is whether Sandra's mom or Keanu's brother understood what was told to them.
I'm sure each one of us has an unusual side to our personalities. Weird thoughts, weird dreams, weird desires, weird characteristics. But is there anyone who truly understands our idiosyncracies? Does such a person exist? Oh, sure, our spouses, boyfriends/girlfriends, relatives, friends love and accept us in spite of our quirks. But do they truly understand us? Do they really understand what we're thinking, what we're feeling? Do they empathize or do they just "accept"? And is that really so important?
In the greater scheme of things, it might not seem so important. After all, having all these people around you, loving you, supporting you, I mean, what else could you ask for? They're there for you through thick and through thin, sharing with you the laughter and the tears and everything else in between that fate throws at you. But sometimes you find yourself wondering if that singular person out there exists, someone who not only accepts your weirdness, but understands why you find solace in your weirdness and that he/she feels or has felt similarly. You know, someone who just doesn't say "That's okay. Whatever you're going through, I'm here for you." Or, "Oh, yeah, I understand" then goes totally off-tangent on you and narrates a marginally related experience (kind of like what I did here with The Lake House). But someone who can say with simple sincerity and conviction, "I understand you"--and you know you ain't being BS'ed, does such a person really exist?
I'm not talking about support groups, because obviously members of these groups have common experiences that make each one relate to the others. I'm referring mostly to people's immediate circle of family and friends. Because in spite of being surrounded by people who love you and whom you love, it's entirely possible to feel lonely even when you're not alone.