The very short version: It's a tradition.
The short version: It's a Jewish tradition.
Disclosure 1: The following text represents only my own personal view of this tradition.
Disclosure 2: I am not lazy, I actually prefer to shave. In fact, the beard stops itching only a few days before I shave.
The 33 days between the first day of Passover and Lag BaOmer is a period of mourning in the Jewish tradition. The visible manifestation of this mourning is the Jewish custom to avoid cutting your hair and in particular not to shave. The aim of this page is to tell (my perspective of) the story of this custom (and the reason I observe it). Whether this story is a hyperbole, a parable or an ellipsis is a matter of taste; however, the story caries a powerful message that resonate through Jewish and human history.
This story dates back to the days of the Roman empire and in particular to the Bar Kokhba revolt in 132-136 CE. The Bar-Kokhba revolt was the third, and last, Jewish revolt against the Roman empire. Simon Bar-Kokhba was the "prince" of the provisional independent Jewish state established in the revolt. However, the most prominent spiritual leader of the Jewish people at that time was Rabbi Akiva. One of the spiritual drives of the rebellion was the belief, indulged by Rabbi Akiva, that Bar-Kokhba was the Messiah.
In the days preceding the rebellion, according to the tradition, Rabi Akiva had some 24,000 students or 12,000 chavruta (pairs of Torah study partners), all of whom joined the rebellion. The root of the mourning period observed by Jewish people between Passover and Lag BaOmer is the disrespect those students have shown to one another usually referred to in Hebrew as "Sinat Chinam" (Useless/Baseless Hatred). The common theme of all these stories is that God punished Rabi Akiva's students for their behavior, causing the death of most of them, in this period of the year and the punishment have ended on Lag BaOmer after they've repented. The nature of the punishment is different in the various stories, sometimes it is a a plague of Diphtheria and sometimes a massacre by the Roman legions, however the moral is always the same - they died as a punishment for Useless/Baseless Hatred.
According to Jewish lore, this is not the first time God tried to teach the Jewish people this lesson. Not 50 years prior to the Bar-Kokhba rebellion, the Great Rebellion (see ) ended in the destruction of the Second Temple. One can argue that the rebellion was doomed to begin with, but one of the reasons that the Siege of Jerusalem ended after only 6 months was the burning of the food stores by the Zealots (an extreme and violent sect of the Jewish people) as part of a civil war within the city taking place while Roman legions were pouring down from Phoenicia.
Once again, to claim that the death of the students of Rabi Akiva or the destruction of the Second Temple is due to divine intervention is a matter of belief, but I feel the message resonating from these stories is universal. This mourning period is a good time to reconsider the way we think and treat other people, whether they are strangers or not. This message seems to be even more relevant in the times of social media when people, common and famous, can publish, anonymously or not, hateful and offensive messages in a keystroke.
 With the exception of a few particular days: the seventh day of passover, Saturdays and Israel's indepedence day.
 Other customs are: not conducting marriages, avoiding instrumental music, avoiding parties etc.
 An interesting discussion about the connection between geometry and rhetorics can be found here.
 The Bar Kokhba revolt is also known as the third Jewish-Roman War or the second revolt of Judia. The first two Jewish revolts are known as the Great Revolt which took place, like the Bar-Kokhba revolt, in the province of Judea. The second Jewish revolt is known as the Kitos War (also known in Jewish lore as the rebellion of the diaspora) which took place in several places in the middle east such as Egypt, Cyprus and Cyrenaica but notably not in Judea.