A vessel has walls which shut out the whole world and thus it is able to receive the amount it can hold. HY is Ain Sof - infinite - anything and everything that He desired to be able to seen in Him is going to be just a vessel which HY empowers the walls of this vessel to shut out His infinite presence, so that the vessel can actually show something particular to itself, what it contains. This is true at every level, even the highest height - the main thing to achieve is the shutting out of the Infinite so that one can attain what he is focused on.
In Likutay Moharan (#49), Rabbainu revealed further that a Jewish heart is Ain Sof, and from there on a person must build the constraints. That means a person already has such holiness, even higher than all the greatest tzaddikim ever, except that he is so overwhelmed and swamped that he is lost and unaware and detached, and in all likelihood he will find some tiny - probably bad - idea and latch on to it, and completely derail himself GF from the infinite and imprison himself GF in a vanity - mirage - of this world. By studying the Torah and screaming out to HY, one concentrates his focus and builds the walls of his vessels to shut out - even the immense light and goodness - until he can successfully achieve clear perception of holy concepts B"H.
This is what Rabbainu says in LM Torah 5, that the main thing is Yira - fear - which is the vessel, and even though the Ahava - love - is stronger, the Ahava comes automatically with the Yira. As soon as one has the vessel, the Love is already filling it up.
When a person is to receive a sum of money - a check - he needs a bank account, otherwise the check doesn't mean anything to him.
The currency is da'as (realization of knowledge) and the vessel is fear - Yira. [This is the secret of the revelation in the Ain Sof, the secret of AHYH ashehr AHYH].
If a person doesn't go out to the fields and communicate with HY - he doesn't have the vessel - he has no way of hearing HY. So unless he does some unbelievable act of G-dliness which will merit him to a complete metamorphosis - that HY gives him a transformation of special vessels so that he can even experience and live the Divine in his regular life - he doesn't stand a chance of catching even a ray of light from HY. He simply is not plugged in to the circuit.
A teacher gave out a test. Before starting the test the teacher instructed to turn the test over - with the blank side up, and then let the class proceed - setting a two hour time limit to finish. The teacher never told them to turn the paper back over, but most people do just that, they don't follow the real instructions, and they might answer some or even all of the questions - but they failed miserably.
The Ramchal says that all the yichudim - unifications/kabalistic meditations - must be done in the field - they simply aren't designed to work in a house or settlement. Most people don't pay attention to this - if they go to the field, then how can they proceed with the devotions. But the real devotion is to follow the real instructions.
There's a great deal more to be said, but B"H at least I got all of this down.
Na Nach Nachma Nachman MeUman!