The reason there are so many diverse subsections in physics is because physicists study the universe and everything in it, from the very small (quantum) to the very large (cosmology). It is exactly this large scope of learning that excites prospective physicists; they are afforded the opportunity to study a host of different areas before focusing on a subdivision in which they can become experts.
Physics has compartmentalized different fields so that the research of different areas of the universe is more manageable. For instance, astrophysicists apply their knowledge of principles of physics to try to understand the origin of stars, planets, galaxies, and other components of space. Atomic physicists conduct research into atoms; a different section of physics.
The important thing to remember, no matter which area of physics excites you, is that being a physicist requires commitment and diligence. An insatiable appetite to understand complex concepts and to comprehend the relationship between energy and matter should be inherent in any person wishing to pursue a career in this field.