UML aggregation vs association

Maybe this can help you, but i don't think you will find the perfect explanation:

The difference is one of implication. Aggregation denotes whole/part relationships whereas associations do not. However, there is not likely to be much difference in the way that the two relationships are implemented. That is, it would be very difficult to look at the code and determine whether a particular relationship ought to be aggregation or association. For this reason, it is pretty safe to ignore the aggregation relationship altogether.
[Robert C. Martin | UML]

And an example for each situation:

a) Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with a single teacher and single student can associate with multiple teachers, but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently.

b) Aggregation is a specialized form of Association where all object have their own lifecycle but there is ownership and child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belong to multiple departments, but if we delete the department, the teacher object will not be destroyed. We can think about “has-a” relationship.
[Maesh | GeeksWithBlogs]

I tend to use Aggregation to show a relation that is the same as a Composition with one big distinction: the containing class is NOT responsible for the life-cycle of the contained object. Typically, a (non-null) pointer or reference to the object-to-be-contained is passed to the containing class's constructor. The containing object, for the duration of its life-cycle, depends upon the contained object existing. The containing object cannot do its job (fully) without the contained object. This is my interpretation of the "Part/Whole" relationship implied by Aggregation.

Rumbaugh's statement is the most telling and Uncle Bob's good advice. As I've said elsewhere, Aggregation is semantically so weak as to offer nothing practically beneficial. It only has one valid corner case (acyclicity of recursive relationships) however few people know and understand that. So you end up having to point out in comments anyway.

I just don't use it. And have never felt any loss. Stick with simple binary associations and focus on what really matters - getting the cardinality and naming right. You'll get far more from that than trying to decide the undecidable association vs. aggregation.

hth.