Jeremiah 17:5-8

'Cursed is the man who trusts in man
And makes flesh his strength,
Whose heart departs from the LORD.
For he shall be like a shrub in the desert,
And shall not see when good comes,
But shall inhabit the parched places in the wilderness,
In a salt land which is not inhabited.' (Jeremiah 17:5-6)

When we think ourselves the most important, when we trust ourselves, all that can come of it is ruin. If our strength lies in our own flesh, then what hope do we have? Additionally, these verses make clear that trusting in your own strength causes you to miss the times when good comes. In other words, you are too busy thinking that you're doing okay by yourself that you miss when God does something truly good for you. Such thinking is like inhabiting a parched, dry land where nobody lives. For if God is the fountain of living waters (2:13, 17:13), then to forsake Him in favour of yourself is to live in a dry and thirsty land. Do we feel this? I'm guessing that all of us have had times where we have trusted our own strength for a period of time, rather than resting in God's. And for a while you feel good, until you realize that all you do isn't achieving anything, everything is hollow and worthless, and when trouble comes, your own strength fails anyway.

'Blessed is the man who trusts in the LORD,
And whose hope is the LORD.
For he shall be like a tree planted by the waters,
Which spreads out its roots by the river,
And will not fear when heat comes;
But its leaf will be green,
And will not be anxious in the year of drought,
Nor will cease from yielding fruit.' (Jeremiah 17:7-8)

Conversely, when we put our trust completely in God, everything we do achieves something, everything has meaning, and when trouble comes we pass through it stronger than what we went into it. We aren't anxious and wanting when not much is happening, but we have patience in the LORD.
Let us all learn to patiently trust the LORD in everything we do, and rely on His strength to get us through day by day, rather than our own skill or ingenuity or any other ability we have (by God's grace).

As a postscript, note that this passage states the negative first - 'Cursed is the man who...', then after this negative, brings in the positive, 'Blessed is the man who...'. Do not be fooled into thinking that we must always be positive and never say anything negative (for it will only scare people away!). God uses this method frequently, and effectively, and so should we. But remember, the positive is so much greater than the negative!