God wants us to be realistic
We must not rely on miracles. God runs our lives through the laws of nature, and therefore we need to make a reasonable effort that will give God a seemingly natural way to bring about our success. Even dramatic, nature-altering miracles were done, as much as possible, through natural means. Even in the greatest miracle in history, in splitting the sea, God first brought a strong wind to churn up the waters and then caused the sea to split. Similarly, we have to make sure we are investing enough effort so that our success will not appear to be miraculous.
God wants us to learn to be responsible
God wants us to be responsible, because His overall desire for is that we become close to Him by emulating Him. One fundamental way in which we do that is by taking responsibility, just as God, as Supervisor and Sustainer of the world, is responsible for everything that happens in the universe.
Granting us success without compelling us to take responsibility would be diametrically opposed to God's purpose, because doing so would make us childish and self-centered. That would cause us to be unlike God and distant from Him.
This applies to parents as well. If we give our children everything they want without expecting them to take responsibility, we will retard their emotional and spiritual growth and impair their relationship with God.
In every aspect of our own lives, we need to take the appropriate amount of responsibility. By doing so, we emulate God and become closer to Him.
However, as important as taking responsibility is to our development as human beings, it is a form of arrogance the think that we succeeded because we took responsibility. Rather, we need to view our taking responsibility as the prerequisite that make it possible for God to bless our efforts with success, because now success won't corrupt us. Taking responsibility is not why we succeeded; it is why God gave us the gift of success.
When trust in God is used as an excuse to avoid taking responsibility, it is a terrible distortion of what true trust in God is.
Too much effort is also a mistake
Investing too little effort is irresponsible, but investing too much effort can also be a mistake. The proper amount of effort is different for each individual and is dependent on one's current level of trust in God. A possible rule is that if something appears unrealistic and irresponsible to you, then for you it probably is, and therefore your current level of trust requires more of an effort from you.
It is easy to delude oneself in this regard, by mistaking laziness for bitachon. One must be realistic and not use faith and trust as a loophole to escape from obligation.
Source: Rav Noach Weinberg, as quoted by Rav Yitzchak Coopersmith in "Constant Connection".