While everybody observes certain mourning customs between the beginning of sefirat ha'omer and lag b'omer (such as refraining from shaving, listening to live music, etc.), some people continue to observe the mourning customs after lag b'omer, while others do not. What are the reasons for continuing to observe the mourning, and what are the reasons for stopping at lag b'omer?

In my observation, most people observe certain mourning customs between the beginning of sefirat ha'omer and lag b'omer (such as refraining from shaving, listening to live music, etc.). Some people continue to observe the mourning customs after lag b'omer, while others do not. What are the reasons for continuing to observe the mourning, and what are the reasons for stopping at lag b'omer?