Is simple misunderstandings.
The root cause of the conflict in Sri Lanka is just that- one big misunderstanding.

You take something embarrassingly simple- co-existence of two races in harmony on shared land- and add layers upon layers of complexity that you no longer see the core issue.

I will be so bold (you may say naive) as to summarize the conflict in Sri Lanka in a few simple annotated statements/claims.
(s1) The minority Tamils in Sri Lanka were (claimed to have been) discriminated against by the majority Sinhalease.
After independence from the British, in addition to English, Tamil’s have had to learn the Sinhala language.
I hope this list can be more ably filled by some Tamil reader friends with first hand accounts of such discrimination …

(s2) There have been several violent Sinhala-Tamil riots which strengthened the separatist cause.
Most notably, the one in 1983, where over a thousand Tamil civilians were massacred by organized Sinhalease thugs.

(s3) LTTE terrorists (supported by donations from expatriate Sri Lankan Tamils) are fighting a bloody terrorist war against the government- at terrible cost to both Sinhala and Tamil civilians- with the end-goal of carving out a separate ethnically pure Tamil state.
The LTTE started small as a group of organized Tamil thugs, but now is the “self-proclaimed sole representative of the Tamil people” by virtue of killing most of the moderate Tamil leaders.
The LTTE have killed more Tamils than, the total number who died during the 1983 Black July and since.

(s4) Expatriate Tamils, who support the LTTE, believe that in a unitary Sri Lanka, sans LTTE terrorists (as a balancing factor), Tamil civilians will continue to be discriminated against.
But Sri Lanka of today is not the Sri Lanka of 1983.

(s5) The party politics of Sri Lanka, where the concept of national solidarity is alien to the opposition, has created a favorable situation for the separatist cause.
Terrorists can “bora diye malu banna”, or “fish in murky waters”- this is the saddest part of all.

Above 5 statements/claims summarize the layers of complexity surrounding the core issue.
But the main problem that need to be solved is:
If possible, to establish the political, administrative, and judicial structure so that the “peaceful co-existence of Sinhala and Tamil races on shared land” is possible.

Contrary to what the international community believes and what some expatriate Tamils believe, IT IS POSSIBLE.

Proof: Colombo.

If there are any doubters come down to Colombo and talk to ordinary Tamil and Sinhala civilians.
Observe how Sinhalease and Tamils interact in work-places, in Schools and Universities.
Observe Sinhalease and Tamils who are neighbors.
Observe Sinhalease worshipping shared deities- God Kandasamy, God Visunu, God Shiva, God Ganesh…
Observer life as it is.

Because the root of enlightenment is true understanding.