It's evident that dreams are a product of what already exists, as I explained previously. They don't remove us from the current world, instead, they help us understand it. Dreams are explainable, they come from reality, and they open up the door to the unconscious mind. Since this is the case, whatever causes a dream determines the approach we should take to understanding it.
1. The External Objective Sensory Stimuli.
Have you ever woken up to the beeping of an alarm clock and noticed the beeping was in your dream? Or heard a conversation while asleep, but realized it was also a part of your dream? It's not uncommon that such things occur, and it's proof that even during sleep our minds are in tune with our surroundings. If a light is shined in your face, you'll wake up. The same thing happens if there's a strong scent, a change of temperature, or any sign of discomfort.
All of this gives a glimpse of the first cause of dreams: external objective stimuli.
"A sensory impression is recognised by us and correctly interpreted, i.e., it is classed with the memory group to which it belongs according to all previous experience, if the impression is strong, clear, and long enough, and if we have the necessary time at our disposal for this reflection. If these conditions are not fulfilled, we mistake the objects which give rise to the impression, and on it's basis we form an illusion." - Freud
If it doesn't wake us up, it will somehow blend into the dream if the sense is strong enough. In these cases where it doesn't wake us up, our brain will have to replace it with something else that relates. (For example, the beeping of the clock might be the ringing of the church bell in a dream.)
2. The Internal Subjective Sensory Stimuli.
Do you count sheep? I tried it once, didn't work for me. What often happens in the process of falling asleep is where we find the main proof of this particular stimulus. It's the imagery: the hypnotic visuals which counting sheep is meant to trigger. That's also the reason it doesn't work for me - I have to think too hard. Instead, I allow scenes to play in my mind naturally, and I do this by imagining scenarios and putting myself into a state where this happens automatically.
"The subjective sensory stimuli as a source of the dream have the obvious advantage that unlike the objective stimuli they are independent of external accidents. They are, so to speak, at the disposal of the explanation as often as it needs them. They are, however, in so far inferior to the objective sensory stimuli that the role of dream inciter, which observation and experiment have proven for the latter, can be verified in their case only with difficulty or not at all." - Freud
These hypnotic visuals we encounter while drifting off to sleep are similar to an overture announcing an opera that's about to begin. The thoughts that go through our mind play a part in forming the dream that follows. While the dream may take a different turn, the visuals are initially what cause the dream to form.
3. Internal Organic Physical Excitation.
“...the history of a woman of forty-three years, who, during several years of apparently perfect health, was troubled with anxiety dreams, and in whom medical examination later disclosed an incipient affection of the heart to which she soon succumbed.” - Freud
This stimulus is similar to the external sensory stimuli because they're both physical. The only difference is that this one deals with what happens within our bodies, not outside.
It's important to realize that our dreams will only adapt to stimuli if they are strong enough - just like with the external objective stimuli. The same applies on the internal side of our bodies, so unless something is unusual, we shouldn't be dreaming about the inside of our bodies.
According to Freud, someone with lung issues might dream about suffocating, being crowded, or flight. If something internal is disturbing our dreams, there may be a physical explanation for it. Again, if our organs were in a perfectly healthy state, there would be no reason to dream about them.
4. Purely Psychical Exciting Sources.
There's some debate about whether dreams are a result of everyday events or our desired interests. Do dreams take us away from the normal patterns of life, or do they tend to focus on our personal mysteries? When it comes to the pure psychic stimuli, the question is mainly regarding whatever is on the mind of the dreamer during the awakened state.
To further clarify the debate here, let's take a look at some specific examples. Does a construction worker dream about construction? Or, does he dream about his other interests that he wishes he spent more time pursuing?
There's a pattern throughout the other three stimuli that I think is important to recognize: whenever something unusual, different, or new occurs, it triggers an effect in the dream. Let's take that same line of thinking and apply it here as well.
With that in mind, it seems more likely that he would dream about those other interests. That's not to say he can't ever dream about construction or that those interests have to be considered priorities - those interests can be anything. It's very likely he'll dream about construction, but only if something significant happens. For example, maybe one day on the job he drops his hammer - but he hardly ever does so! If that event sticks in his mind, his psyche will most likely regard this as unuseful information and as a result, something similar might appear in his dream.
At this point, we have the basics covered for interpreting the dream. As Freud explains that once we combine the conscious interests with both inner and outer stimuli, "... the riddle of the dream sources would thus be solved, leaving only the task of separating the part played by the psychic and the somatic dream stimuli in individual dreams." The problem is that this method is difficult to pull off and the information we have must be put to use in some other way. But, that will have to wait until some other time.