What is easy to get into, but hard to get out of?

“A car”

While it's easy to get into a car, it's not inherently hard to get out of it unless there are specific circumstances, like being trapped.

“A relationship”

Although relationships can be complicated, they are not universally hard to exit from, as it varies greatly by individual circumstances.

“Debt”

While debt can be hard to manage and exit from, it is not as universally applicable as 'trouble' and doesn't fit the phrasing of the riddle as neatly.