"Becoming" (bhava) means "the sphere of birth." Sensual desire is born at sights, sounds, tastes, smells, feelings and thoughts, identifying with these things. The mind holds fast and is stuck to sensuality.
Some cultivators get bored, fed up, tired of the practice and lazy. You don't have to look very far, just look at how people can't seem to keep the Dhamma in mind, and yet if they get scolded they'll hold on to it for ages. They may get scolded at the beginning of the Rains, and even after the Rains Retreat has ended they still haven't forgotten it. Their whole lives they still won't forget it if it goes down deep enough.
But when it comes to the Buddha's teaching, telling us to be moderate, to be restrained, to practice conscientiously... why don't people take these things to their hearts? Why do they keep forgetting these things? You don't have to look very far, just look at our practice here. For example, establishing standards such as: after the meal while washing your bowls, don't chatter! Even this much seems to be beyond people. Even though we know that chattering is not particularly useful and binds us to sensuality... people still like talking. Pretty soon they start to disagree and eventually get into arguments and squabbles. There's nothing more to it than this.
Now this isn't anything subtle or refined, it's pretty basic, and yet people don't seem to really make much effort with it. They say they want to see the Dhamma, but they want to see it on their own terms, they don't want to follow the path of practice. That's as far as they go. All these standards of practice are skillful means for penetrating to and seeing the Dhamma, but people don't practice accordingly.
To say "real practice" or "ardent practice" doesn't necessarily mean you have to expend a whole lot of energy -- just put some effort into the mind, making some effort with all the feelings that arise, especially those which are steeped in sensuality. These are our enemies.
But people can't seem to do it. Every year, as the end of the Rains Retreat approaches, it gets worse and worse. Some of the monks have reached the limit of their endurance, the "end of their tether." The closer we get to the end of the Rains the worse they get, they have no consistency in their practice. I speak about this every year and yet people can't seem to remember it. We establish a certain standard and in not even a year it's fallen apart. Almost finished the Retreat and it starts -- the chatter, the socializing and everything else. It all goes to pieces. This is how it tends to be.
Those who are really interested in the practice should consider why this is so. It's because people don't see the adverse results of these things.
When we are accepted into the Buddhist monkhood we live simply. And yet some of them disrobe to go to the front, where the bullets fly past them every day -- they prefer it like that. They really want to go. Danger surrounds them on all sides and yet they're prepared to go. Why don't they see the danger? They're prepared to die by the gun but nobody wants to die developing virtue. Just seeing this is enough... it's because they're slaves, nothing else. See this much and you know what it's all about. People don't see the danger.
This is really amazing, isn't it? You'd think they could see it but they can't. If they can't see it even then, then there's no way they can get out. They're determined to whirl around in samsara. This is how things are. Just talking about simple things like this we can begin to understand.
If you were to ask them, "Why were you born?" They'd probably have a lot of trouble answering, because they can't see it. They're sunk in the world of the senses and sunk in becoming (bhava).  Bhava is the sphere of birth, our birthplace. To put it simply, where are beings born from? Bhava is the preliminary condition for birth. Wherever birth takes place, that's bhava.
For example, suppose we had an orchard of apple trees that we were particularly fond of. That's a bhava for us if we don't reflect with wisdom. How so? Suppose our orchard contained a hundred or a thousand apple trees... it doesn't really matter what kind of trees they are, just so long as we consider them to be "our own" trees... then we are going to be "born" as a "worm" in every single one of those trees. We bore into every one, even though our human body is still back there in the house, we send out "tentacles" into every one of those trees.
Now, how do we know that it's a bhava? It's a bhava (sphere of existence) because of our clinging to the idea that those trees are our own, that that orchard is our own. If someone were to take an ax and cut one of the trees down, the owner over there in the house "dies" along with the tree. He gets furious, and has to go and set things right, to fight and maybe even kill over it. That quarreling is the "birth." The "sphere of birth" is the orchard of trees that we cling to as our own. We are "born" right at the point where we consider them to be our own, born from that bhava. Even if we had a thousand apple trees, if someone were to cut down just one it'd be like cutting the owner down.
Whatever we cling to we are born right there, we exist right there. We are born as soon as we "know." This is knowing through not-knowing: we know that someone has cut down one of our trees. But we don't know that those trees are not really ours. This is called "knowing through not-knowing." We are bound to be born into that bhava.
Vatta the wheel of conditioned existence, operates like this. People cling to bhava, they depend on bhava. If they cherish bhava, this is birth . And if they fall into suffering over that same thing, this is also a birth. As long as we can't let go we are stuck in the rut of samsara, spinning around like a wheel. Look into this, contemplate it. Whatever we cling to as being us or ours, that is a place for birth.
There must be a bhava, a sphere of birth, before birth can take place. Therefore the Buddha said, whatever you have, don't "have" it. Let it be there but don't make it yours. You must understand this "having" and "not having," know the truth of them, don't flounder in suffering. http://www.what-buddha-taught.net/Books/Ajahn_Chah_Food_for_the_Heart.htm#flood