Let's discuss solipsism, what it means, and how life changes if you look too closely.
The basic definition of solipsism is "the view or theory that the self is all that can be known to exist". Most people immediately assume this means that everything outside the self is a figment of imagination, an illusion, or a creation of the mind. Such a view is rightly called extreme, as not knowing something certainly does not mean that the unknown doesn't exist at all--though that could also be true!
Solipsism implies that the contents of your own mind (e.g. your self) are the only things you have the ability to know. This is essentially Cartesian ("I think therefore I am") in that your mind acts as the organizer for experience, and the experience so organized to your own mind represents the entirety of self. You cannot know what I think, feel, or dream, nor can you see what the world looks like to a dog. You and I are trapped within the confines of our own view.
There is literally no way to verify anything beyond the limits of your perception. In order to know if someone is in pain, you need to form a picture in your mind (symptoms, blood, facial expressions, verbal acknowledgment) which becomes your mental assessment if this person requires medical attention or not. You know someone is happy because they laugh, seem jolly, or simply say they are happy--all of which become part of your experience, your mental picture, of the world.
Your world. Does everyone see the world in exactly the same way? Who knows, really, since we can't be that person to find out. Even if you downloaded all their sensory data, every experience of their life, would it feel the same way to you? Would you laugh or cry at the same moments? Do you get excited about another person's sexual partner? How can we think and feel the same things about every experience?
Even on this rudimentary level, we know each one of us has a slightly different view of things, even if we witness the same event. Rationally, this implies that reality exists within the mind, isolated in its understanding from all other minds. It seems self-evident that this is so.
What does it mean, though? Well, for one, it implies that every human being experiences life alone. Other people, places, and events shape our view of the world, but we experience the journey as a unique collection of sensory stimuli and reflective moments. This information comes from an external source, doesn't it?
A vast majority of who we are, what we do, and why we do things is motivated by external forces: what others expect, social norms, traditions, laws, and so on. These larger forces create group behaviors; combined with underlying human instincts, it appears that a lot of our world view is built atop the collective behaviors of previous generations.
As a starting inquiry, as yourself how much of who you are, what you do, and what you think about is a result of environmental factors. Especially in the West, we stress individuality while liking this person's book, that song, or some new fashion--we collect ideas from all around us. How much of this is a result of our own thought? Ordering something different from the menu is still ordering from the menu, isn't it?
What are you, then? Was Descartes correct in asserting that our only foundation of existence, the only path to knowing self, is to understand "I think therefore I am"? No matter what we are aware of, what forms our mental picture of reality, we are conscious, thinking beings. All the experiences, everything in the world around us, has no meaning until we think it does.
Here's a video I put together a while back in which I discuss solipsism and what it means.