There many colors to each word, depending on individual perception and situation. I dont recommend reading Buddhism-related material (life is suffering) without having the practice to go with it. Enlightened texts serve as a guide to those attempting to approach that stage.
To answer you question, life is considered suffering because of two things. One, since a person who is unawakened (that is, in a fully concsious state, not self-actualized) is considered to be in perpetual suffering from everyday lives, chores and people. One must practice deep meditation, contemplation or zazen in order to "wake up" and realize suffering to be an illusion. The second and more practical reason that life is suffering is that its a learning ground, without suffering there is no intellectual growth, adaptation, life lessons and memories. Some people even prefer suffering over non-suffering because they realize the importance of progressing. That is not to say that these peple literally like to suffer pain but instead indicates that one's perception is relative to the mindset of the individual. If you believe you are suffering, you are, if you believe you are not, you will not be.
Boris R.
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