At first glimpse, the prophet Jeremiah seems at least heartless if not just plain cynical.
Cursed is the man who trusts in human beings
In fact what the prophet is really showing us is the path to true love. If we, as he writes,"seek our strength in flesh", then we will seek out relationships that can be the source of that strength. The most overt form of this most of us have experienced-- the person who only calls when they need something.
The prophet Jeremiah is not saying we shouldn't trust people or that if we trust people we are always going to be disappointed. What is translated here as "trust in" might better be phrased as "be dependent on." As people of faith we should be dependent on God, find our strength in God alone.
When we depend only on God then we are free to truly love. We can love others not for what we get from the relationship, but love them just for who they are. Love can then be not about me getting something, but me giving. When we truly love we are focused on giving something of ourselves to another. if they love us, they in turn give something of themselves to us. In real love it is free gift. We don't demand it. We have no right to expect it.
How many people miss out on the joy of life because they mistakenly believe that there is some human relationship that they NEED that they don't have. If they could just find the right person, they think, then they would be happy. This is what Jeremiah is warning us away from. The only relationship I truly need is God. My very life, my very existence comes from God. Without God I do not have life.
Every human relationship should be seen as pure gift, an opportunity to share what I have received from God with someone else, an opportunity to love.