Just about everyone thinks they have plenty of time left in life. The only ones who don't think this way are those with a terminal disease in its latter stages. And, to be honest, they are the ones who are living like we all should live - taking advantage of every moment that they have - because they know it may be the last one they have to live.
Boasting about tomorrow is the problem in this passage. It is the attitude that says, "I not only have tomorrow - I can make of tomorrow anything I want of it!" Pride is the reason we would boast of tomorrow. We do it because we think "we" determine our future and that "we" can make it what "we" want. When this attitude prevails in our lives it also carries with it that the only reason to live is for this world and what it can bring. James speaks of this attitude when he writes, "Come now, you who say, Today or tomorrow we will go to such and such a city, and spend a year there and engage in business and make a profit. Yet you do not know what your life will be like tomorrow. You are just a vapor that appears for a little while and then vanishes away. Instead, you ought to say, "If the Lord wills, we will live and also do this or that. But as it is, you boast in your arrogance; all such boasting is evil." (James 4) This is all about whether we rule our lives - or if God rules them. And the fact is that God not only rules our lives - but that He is sovereign over all creation.
We don't know what a day may bring forth. Here is the second reason why this attitude of boasting in a day is prideful. When we do we are acting as if we are omniscient and omnipotent. We think we have the power to make whatever we want to happen - when the truth is that we cannot make anything happen that is outside of God's ultimate purpose and plan. We also think we know tomorrow when the fact is that we don't even know what the next 10 minutes hold. As James said, our attitude needs to be, "If the Lord wills, we will do this or that." Anything other than this is sin.