This link is second only to sankhara in giving translators and students of Buddhism trouble. The examples of the field are not much help to us because they are embedded in their time and place, making reference to things that are no concern of ours now. This is not the same as saying that they are meaningless to us. Two of the three spheres get mentioned in relation to levels of meditation, and it is clear that some of the people of the time felt that when they reached these states they had reached “that other world”, so these represent the desired outcome from leading a good life: go to the world where one takes form (and lives happily ever after in bliss), or go to the world where one becomes totally absorbed in Union with the Great All (and forever after experience bliss), or — the third option seems likely as — simply return to the world of the senses (and hopefully enjoy sensual bliss).
The three “spheres” are various conceptions of where the self goes after death: back into the world of the senses, taking form in the world beyond, or being diffused into union with brahman, and though you and I may not have these particular conceptions about what happens to us after we die, each of us has our own variants on these beliefs. We might believe that when we die we are just gone, our constituent parts dissolved and diffused into the matter of our world in its place as part of the universe. Some might believe that when one dies one goes on to heaven (if we are good) and that would be comparable to living happily ever after in bliss in company with the deities of the day. Others may believe that we reincarnate into a new life. It really doesn’t matter what form our beliefs take, they still shape our sense of self, and how we behave in the world. Any sense of certainty about what happens after we die is liable to be based on far too little information, and yet we do tend to develop certainty. Even without concerning ourselves with what happens after death (for years I conceived of myself as an open-minded agnostic with no opinion on the subject) we still tend to perceive of ourselves as having a lasting self for the duration of this life at least, and it is our nature to at least hope, at some level, that that self will survive death, or at the very least hope that that self has a meaning that will outlast our short stay here on the planet.
If you object to the above statement, feeling it doesn’t apply to you, remember, first, that we are talking about conceptions of self that are founded on ignorance. If you are a practicing Buddhist who has some understanding of what the Buddha taught, you may have less of a tendency to hold beliefs about having a lasting self than many folks do, because you’ve been paying attention, and learned skills to look for that self — and you’ve probably not found it. But even so (if you are like me) you may find as you keep probing that there are little pockets of belief you hadn’t known were there (it was not until I realized how uncertain my fate after death actually was that I discovered I was not as agnostic as I had thought — I was still holding out hope that I would somehow survive).
What “becoming” is pointing out to us is the ways in which our opinions about who we are get shaped into beliefs about who we are, and how those beliefs transform us into (the next step’s) visible “self”; this is what we should be looking for, because those opinions are the field in which beliefs grow, and the beliefs blossom into something else entirely, as the next step shows us.
With the definition of this link given in terms of “becoming into a certain sort of world” — and if we combine it with the idea of “nutriment” — it might be said that “we are what we eat” (or at least “we *become* what we eat). The fuel of our opinions about who we are and how the world operates feed our sense of who we are; and the way we sort our experiences in terms of what-is-like-us and what-is-not leads us to a false certainty that the world is just as we perceive it to be. We can say, then, that because we believe the world is this way, we will see that it *is* this way. If we expect to arrive in a certain world, we will, because what we notice about the world will tend to support our perception of it being as we expect (we will ignore or block out information that contradicts our views). This is how it happens that we will be “born into a world of our own making”.
On the other hand, is the world *really* as we perceive it? We all know that it really isn’t. The difference between the two — our perception and the reality — will be brought home to us as the results of our behavior. If our perceived world is *too* far off from the way the world actually works, our happy expectations of the world working the way we think it should will get bumped up against reality, and we’re going to feel it. (An example of the Buddha talking about this can be found in the sutta about The Dog Duty Ascetic, in which the believer in the title is advised that diligent effort will have him end up in the company of dogs, as long as his aspirations are just that humble, but if he thinks it’ll get him into some higher heaven, he’s kidding himself.)
From the midpoint of dependent arising (which is the description of our oft-repeated rituals) to this link and the next, dependent arising is talking about how our perceptions of our self and the world play out, and what we are doing that makes them appear to us the way they do. “Becoming” speaks of a transitional phase, a sense of self taking shape, preparing to “be born” (become visible) in the world. Only with the last step does the conversation change from “how we perceive self and the world, and why” to the results of our conceptions’ effect on our actions.