Parshat Acharei

This is the matter that G-d has commanded, saying: 'Any man or woman from the house of Israel who will slaughter a bull, a sheep or a goat and has not brought it to the entrance of the Ohel Mo'ed (Tent of Meeting) to bring it as an offering to G-d it shall be considered as bloodshed for that person, they have shed blood, and that person shall be cut off from their people." (Leviticus 17; 2-4) While the Jews were in the desert all animals had to be sacrifices on the altar in the Ohel Mo'ed, and after Temple was constructed in Jerusalem, it was prohibited to offer any sacrifices elsewhere. Not only was it forbidden, but this sin carried the most serious punishment of kares, spiritual death. This needs explanation.
Rabbi Aharon Tendler asks the following question. Why should offering sacrifices on a personal altar be forbidden? If an individual desired to offer a thanksgiving sacrifice for example, after surviving danger or the birth of a child, he would have to travel to Jerusalem in order to show their thanks to G-d. Since it could take weeks to make the trip to Jerusalem and back, most people would put off bringing their sacrifices until the next pilgrimage festival, when they had to go anyway. It would take an unusual level of devotion to maintain enthusiasm for performing the sacrifice in this way. The offerings would inevitably change from a free-willed, outpouring of love and devotion to an imposed obligation of service and commitment. Wouldn't it have been much more intense and intimate for an individual to fulfill his desire to acknowledge G-d at the very moment when His benevolent protection and guidance was revealed? Why limit the recognition of our dependency on G-d in the form of sacrifice to a single location, restricted by time and distance? And why is there such a severe punishment? Is a sacrifice at the wrong place and time not better than no sacrifice?
A similar question could be asked about prayer, which has replaced sacrifices. Though we can pray anywhere, we still have designated times and other restrictions. Why is it better to pray with a minyan in a Synagogue, rather than alone on the top of a mountain, acknowledging G-d whilst surrounded by the magnificence of His world? Why must we pray three times a day, regardless of our intent and enthusiasm, rather than those times when we truly feel the connection to G-d? The "by rote" problem that plagues the routine of prescribed daily prayer would certainly be alleviated if we prayed when we felt G-d's closeness, rather than when we are obliged to do so.
The Ramchal writes (Messilat Yesharim end ch. 1): "The essence of a person's existence in this world is solely to fulfil the Mitzvot, the serving of G-d... and that their every action, great or small, be motivated by no purpose other than that of drawing near to G-d". One of the greatest challenges with which we are faced is to ensure that our religious observance is in order to serve G-d, and not merely to make ourselves feel good.
We are sometimes filled with a sense of religious euphoria, or experience a form of revelation that inspires us to draw close to G-d. Though this moment of insight may act as the catalyst to help us perform the Mitzvot, it should not be our motivation for their performance. The challenge is not to act based on our 'needs' or spiritual desires, but rather solely in order to perform G-d's commandments. This is the meaning of the Mishna (Ethics of the Fathers 2; 4) "Perform [G-d's] will as if it were your will, in order that He may do your will as if it were His will...". The goal is to subordinate our spiritual drives to those commanded us by G-d.
This is why we may only bring sacrifices, or pray, according to specific rules of time and place. The true test is not to pray 'when we are in the mood', but rather to channel that energy so that we can pray to G-d in the most appropriate manner. Bringing a sacrifice where and when one feels like it is not worshipping G-d, but succumbing to personal religious desires. This is a form of self-idolisation, which is why the Torah gives the punishment of spiritual death. Performing only those religious rituals which 'suit' us takes us further from G-d, not closer. The inevitable result of this is a lack of spirituality.