A person is only given as much grace as he merits and is able to take without being overwhelmed by.
The concept of grace is one that can have no place in the modern materialistic world view; that strange idea which presumes that what we perceive with the senses or by means of scientific instruments is more or less all there is. It is also hard to fit into a non-theistic form of spirituality though it forms a central part of traditions as diverse as Christianity and Kashmir Shaivism. But, according to most teachers, grace is essential if there is to be any proper spiritual awakening, and without it we would remain in darkness and ignorance despite all the efforts we might make to dispel this. Whether we conceive of it as coming directly from God or through (not from) a teacher who, for us, is able to manifest the divine, grace is the fire that sets ablaze the combustible material that is our purified (through spiritual practice) soul. If it is absent our mind may have all the spiritual knowledge you could want but that knowledge will be just dead matter without the spark of life, and, while it might inform, it will never inspire.
Sometimes this grace comes suddenly and dramatically (obvious example, St Paul), but at others it seeps gradually into a disciple's consciousness so subtly and quietly that he may not even be aware of it until he compares himself now with how he used to be. However it comes, though, it never comes by chance*. If it does then it will not be true grace but a stolen copy, stolen through the use of drugs or certain esoteric techniques perhaps, which may seem to imitate the effects of grace but which will do so in a way that brings no long term benefit to the disciple. But real grace from God or the Holy Spirit must be earned through hard struggle, spiritual discipline and self-purification, prayer, intense and ardent desire for enlightenment and, yes, suffering. The road to true grace is long and arduous though, because there are various degrees of grace, we may receive blessings along the way which provide openings and encouragement.
Grace is often compared to fire or lightning and I think this is an excellent comparison though it does tend to emphasise the dramatic side of grace. It also emphasises its spiritually transformational side but it's not always helpful to focus on that. Let's be realistic. A person may have one or two spiritual experiences in a lifetime, and that's if they're lucky. They may judge themselves to be highly advanced on the path, even 'enlightened' (a non-Western concept probably not suitable for the Western spiritual practitioner), on the back of that whereas what is more likely is that they, as a beginner, have been given a sign that they are following the right course. Neophytes need to be encouraged whereas old hands are usually trusted to get on with it without reward, and that is doubly so as rewards can be distractions. The spiritual path as it is in this world is not primarily about higher states of consciousness but about loving and serving God to the best of your ability in any situation in which he may see fit to place you. He places you where you are most likely to learn what you need to for your spiritual unfoldment, and where you may best assist others. Lessons of love and humility (the two most important spiritual lessons) are not so easily learnt if you are bathed in bliss. Oh yes, you may demonstrate them at that time but what are you like when the bliss wears off and is not recoverable however hard you try? The spiritual field is littered with gurus and teachers who have over-estimated themselves because of an initial grace and who then try to sustain their power and authority through false methods once that has worn off, leading to often unfortunate results both for themselves and their followers.
Grace implies a theistic universe and a God who is a Person. Non-theistic religion can try to squeeze in the concept of grace but it sits uncomfortably with the idea of a universe in which pure being or consciousness is the bedrock of existence. Certainly grace can be conceived of as coming from enlightened masters who are transmitting spiritual energy but these are basically human individuals who are standing in loco parentis, the parent being God. Grace is gift and gift needs a giver and though the giver can be a guru the question arises who gives him the gift he is passing on? His guru. of course, but who gave it to him? Back we go through the lineage until eventually we do get to God, not that eventually is the right word here for all things come from God now, in this present moment. He is both the ultimate and present source of all grace.
Our task, therefore, is to render ourselves fit for grace, and although it may come to us as a result of our own efforts we should not, for all that, regard it as a right. It is always God's gift even when we have earned it by our endeavours. Sometimes, however, it comes as an encouragement and sometimes as a test.The gurus mentioned above have failed their tests but then so do we all until we pass them. That is all part of the process of growing through experiment and experience which is what this world is for.
If you would receive grace, pray. For it is sincere, almost desperate, asking that shows the readiness to receive. You are a vessel waiting to be filled but make sure your vessel is washed clean so it can hold the pure sparkling liquid that will be poured into it without spoiling the flavour.
* Sometimes it can appear to come by chance but that's a different matter.