As roots grow within a pot, they spread out, searching for minerals and water to absorb. With time, the roots take up more and more space within the pot and become crowded, limiting the amount of nutrients and water they can access.
Another problem you may encounter is that the soil can become compacted, keeping it from having access to water or aeration.
If the soil has been infested with insects or has root rot, your plant needs to be treated and repotted in fresh soil. You may or may not need a larger pot in this case.