The Torah writes many times that man in supposed to reach closeness to Hashem. In Devarim it says, "U'bo tidbak.  And you shall cleave to Him." Similarly in the book of Yehoshua the verse says, "U'ledavka bo. And you shall cleave to Him." Both the Rambam and the Mesilat Yesharim write that the ultimate perfection of man is deveikut, cleaving to Hashem. If the existence of a person is for this purpose, it follows that all his actions should serve this goal. Therefore, a person should accustom himself to ask, "Will this action bring me closer to Hashem or not?" Even if it is permitted, if it doesn't lead to closeness to Hashem it should be avoided, as it is automatically bad.  The Sefer Chareidim lists all the positive and negative mitzvot in the Torah. It includes the positive commandment of deveikut,  cleaving to Hashem, as derived from the verse, U'bo tidbak, which enjoins us not to detach from Hashem for even a moment. The verse mentions ahava (love) and deveikut as two commands, "L'ahava et Hashem, u'l'dovka bo." Therefore the Sefer Chareidim counts them as two separate mitzvot. There's the level of loving Hashem, and the higher level of cleaving to Him. Other commentators count one mitzvah and explain that deveikut is the highest form of ahavat Hashem.  

The Shaarei Teshuva quotes the verse in Devarim, "Hishomer lecha pen tishkach et Hashem. Guard yourself from forgetting Hashem." Chazal say that wherever it says hishomer it refers to a negative commandment. We are warned not to forget Hashem even for a moment. Deveikut has many levels. There's an external distant connection. When two people hold a rope and one shakes it, the person at the other end feels movement from afar. A closer level is when two people shake hands. When they sense each other, a stronger connection is formed. That is how a person should feel about Hashem. The verse in Tehilim says, "Ivdu et Hashem b'simcha." Even if a person serves Hashem with joy it's not enough. "Bo'u lefanav bir'nana," He must come before Him and aim to get closer to Him. David Hamelech says in Tehilim, "V'ani kirvat Elokim li tov. Closeness to Hashem is good for me." The best way to achieve this is to connect to Hashem in one's heart, the main place where the Divine Presence rests in this world.

Rav Yechezkel Abramsky once waved his fingers and asked a student sitting next to him, "Tell me the distance between my nail and the skin around it. The student answered, "The nail is attached to the skin around it. There's no distance." Rav Yechezkel then said, "Hashem is even closer to me than that." That is how a person must envision Hashem.

In Tehilim the pasuk says, "Imo anochi b'tzara." At a time of judgment, Hashem is closer to us then in times of peace. Yet it seems Hashem is more hidden, so the person doesn't sense it. Therefore, David Hamelech said, "V'ani kirvat Elokim li tov." I feel close to Hashem even amid the darkness. Rav Chaim Soloveitchik teaches that the neshama has the same letters as the word neshima, breath. The soul is a breath of Hashem and it yearns to return to its source. As soon as the body weakens its hold, it goes straight back to Hashem.

A student once told me that when he first became observant he had the feeling that when He spoke to Hashem it was as if he was talking to someone far away. After a time when he grew closer to Judaism, he felt as if someone was putting out a hand to him. When he got even closer, he felt two hands. And finally, he felt like a baby in an incubator enveloped in Hashem's warm presence.

The closer a person feels to Hashem, the more he reveals the light of the Shechina within himself. In Bereishit the Torah says that man was created b'tzelem Elokim, in the form of Hashem. How can we say this? We know that Hashem has no body. The Nefesh Hachaim explains that here tzelem means a comparison. A person is compared to Hashem. Just as Hashem controls all the powers in the world, He gave man the ability to impact tens of thousands of powers in this world. He can augment their strength and holiness or dim their light. All the world is connected to the actions, speech, and thought of mankind as it says in Tehilim, "Tenu oz l'Elokim," give power to Hashem's creation.

The Ohev Yisrael points out that although Hashem commanded the Jews to build a mishkan so that He could rest His presence there, His main resting place is inside the heart of every Jew. If a Jew fully believes this, his whole behavior should automatically change. The Baal Haturim points out that the words, yoshev ohalim, used to describe our forefather Yaakov, is the numerical value of 410. This is equal to the same amount of years that the Shechina rested in the beit hamikdash.  A person who purifies himself by studying the Torah and keeping the mitzvot merits to feel the light of the Shechina inside him. Our holy avot who did this, shaped themselves into a living beit mikdash where Hashem's presence shone through.