How to develop patience? I want to know how can I develop patience?
What Buddha said on this?
And also what is the reason of being impatient?
In my understanding, it comes back to a central Buddhist concept of "attachment".

We feel frustrated or angry, we get emotional about things, when we have a strong attachment or prejudice about how things are supposed to be.

These prejudices or preconceptions come from some strong experiences we have in childhood, or from some books we trust, people we highly respect etc. They also come from pain, when we did something and got in trouble. In most of these cases we have some basic formula in our head "this is bad" or "this is good" (why - because blah blah blah). And we are so convinced that this is correct, that we feel like we will never change our opinion in the entire life.

So when we see something or someone doing something that goes against this preconception or attachment, then we feel like they go against Truth (or the way we see it anyway). This makes us frustrated or angry or upset etc.

There are multiple ways to develop patience in Buddhism. On the basic level, we just try to calm down. We tell ourselves how bad it would be if we lose control and start yelling etc. But in my experience this does not work as soon as situation gets serious. We just forget everything and jump right back into emotion.

The next method is to be mindful of one's emotions on the body level. This way we can notice ourselves getting nervous far before it gets out of control. So if we develop feeling of presence of body, and always feel all tensions etc., we get this convenient weather indicator that can show a storm coming 5 days in advance (really maybe 5 minutes, but still good). It is much easier to fix e.g. a conversation that goes towards a scandal 5 minutes before it gets out of control, when things are still calm, but you feel it coming.

And then the most powerful method is to see the attachments or preconceptions that give rise to emotion - and learn to let them go. This is the most difficult, because it means you have to learn to see situation from another perspective, and to not assume that you are 100% right. This can be very difficult in the beginning. But at the end this ability makes you practically immune to all disturbances, 100% patient at all times. It is a very helpful feature to have in life.

My teacher said, enlightened person is like a lawyer. He or she sees every situation from all sides at the same time. This helps him to never lose patience and to make better decisions than anyone. The point is, Buddhist patience is not self-restraint and self-torture, it is freedom from single point of view, freedom from attachments.

P.S.

Same exact logic applies to impatience towards desirable objects. Attachment, preconception etc. - except in this case it's a preconception that the object is "good", usually based on some overgeneralization or idealisation. So the method Buddha prescribed for that is to analyze the object and see that it has negative sides and not only positive. This helps to let go of attachment and come back to patience and balance. All the same advice about being mindful of your body to sense the attachment getting out of control, applies here as well. In Buddhism desire and aversion are considered things of the same nature, one is positive and the other is negative but the principle is the same.