Hi Juliet,
I am not sure I completely understand what you are requesting, but I will do my best to answer. I believe you are essentially asking why existing Activites for a Customer don't show up under the Opportunities tab, if the Activities for this Customer were created before the Opportunity. If this is correct, I believe the reason is, when you create an Activity under the Opportunities tab, the Activity is meant to be more directly related to the Opportunity, then to the actual Customer. This is slightly different then when you create an Activity under the Activities tab, these would be meant to relate directly to a Customer. Also, take a look at the wording for the Activities tab from from with the EditOpportunity screen, it is titled "Existing Activities for this Opportunity", so again showing the activity relates more directly to the Opportunity then the Customer.
There is a way to relate existing Activities for a Customer to an Opportunity if this is something you would like to do. If so, go to the Leads and Contacts List, and select the "GoTo" link from any Lead record you see in the grid that you want to do this for. The goto link will send you to the "EditLead" screen, once there, choose the "Activities" tab. Here you have a list of all your Activities for this lead, and even any related Opportunity number if it has already been related. If you find an Activity not currently related to an Opportunity for that Customer, but want to make them related, click the Activity number for the record you want to do this for. This should load the EditActivity screen for the activity we are looking at. On the left-hand side of this screen you should see the "Opportunities" section in "Recent History", any existing Opportunities already setup for this lead contact will show up here (depending on your filter views). At this point, all you would have to do click the "Relate" button beside the existing Opportunity, this will create the relation you are looking for between this specific Customer Activity, and the Opportunity you created for this Customer.
I am not sure this is possible off-hand, but you might consider speaking with a Method Consultant to discuss if there is anyway to automate this for you, so all existing Customer Activities get related to an Opportunity if created. Please let me know if this was helpful, and if I missunderstood your original question, please let me know so I can answer it correctly.
-Ben