she always said that i should not trust people !

she blieves that Trust is something that must be earned. If you’ve been burned or betrayed in the past, then forming a bond of trust with others is much harder.
but i think trust is a vital element that makes society possible. We need to trust that people will generally do their best to maintain social order. Most people don’t go about murdering or hurting random strangers, or break traffic laws in very dangerous and deliberate manner. Most people go to work on time, leave on time, and their employers make sure to pay them on time.
On an intimate level, trust is much, much harder. Shame and guilt keep a lot of people from revealing their deep, dark secrets. We all carry our baggage and often unconsciously project our bad experiences on others to some degree, while they do the same to us. It keeps many people stuck and unable to fully trust their closest friends. Our bad memories force us keep our guard up even when we need be open and vulnerable. Trust begets trust.

I was asking what would happen if we did not trust people. Actually we would die because we would not be sure if the water is not poisoned, if the food is ok, if we can breathe freely etc. So there has to be a level of trust.
When we ask this question I suppose we´ve made already an experience of disappointmnent. So you saw that not every one is trustful. This is what you will learn during your life to trust every one but to decide if something bad or your life. Trust is usually build over a long time.

but she was wrong :

When you commute through a bus/ train or any public transportation you unknowingly trust the driver.

Trust is not equal to happiness always.
Trust is not equal to heartbreak always