The First Noble truth, or the Truth of Suffering, is perhaps the most misunderstood of all the teachings of the Buddha.
It is often summarized to mean that life is suffering, or everything is suffering. Such incorrect summary is the main reason why some people view Buddhism as pessimistic, which it definitely is not.
In Samyutta Nikaya LVI, 11, the Buddha talks about the truth of suffering thusly:
What is the Noble Truth of Suffering? Birth is suffering, aging is suffering, sickness is suffering, dissociation from the loved is suffering, not to get what one wants is suffering: in short the five categories affected by clinging are suffering.
There is this Noble Truth of Suffering: such was the vision, insight, wisdom, knowing and light that arose in me about things not heard before.
This Noble Truth must be penetrated by fully understanding suffering: such was the vision, insight, wisdom, knowing and light that arose in me about things not heard before.
This Noble Truth has been penetrated by fully understanding suffering: such was the vision, insight, wisdom, knowing and light that arose in me about things not heard before.
As you can see, this is far from a flat statement that everything is suffering.
The Four Noble Truths are modeled after the medical practice of Buddha's time (and pretty much of ours). The first step is to observe the symptoms, that is determine just what is wrong with the patient as far as the obvious outwards signs of the disease.
Hence, the First Noble Truth is based on observation. It simply enumerates some obvious symptoms of the problem every living being experiences.
The Pali word generally translated as suffering is dukkha. Its Sanskrit version is similar, it just has more h's. In either language, the primary meaning is pain, yes, physical pain as observed by physicians and experienced by their patients.
But the Buddha did not use it to simply describe physical pain. Rather, dukkha describes that which is incapable of being satisfied. Here are some English words and expressions that either convey or express the idea and the experience of dukkha:
Get the idea? Does any of this sound familiar? Have you ever said any of those, or at least felt like it? If so, then you have experienced the symptoms desribed by the First Noble Truth, and you know that it, indeed, is a truth.
It does not imply we are constantly suffering. It simply says suffering is a fact of life, and we all experience it. It does not matter whether you are young or old, rich or poor, male or female, human or divine, everyone experiences suffering: I do, you do, Prince William does, Bill Gates does, your pet does. Everyone does.
Furthermore, it appears that no matter what we do, we still face pain, suffering, frustration, and disappointment. (Actually, the Fourth Noble Truth shows a way of overcoming it, but here we are only discussing the first one).
Please note the wording, too. The First Noble Truth does not say "I am suffering" rather it states that suffering exists and we all experience it. Nothing less, and nothing more.