When the Schengen refusal reason is "Your intention to leave the territory of the member states before the expiry of the visa could not be ascertained", it means your application failed to clear a risk assessment.
The procedures for a risk assessment vary from country-to-country and are never publicly disclosed. Generally, however, they take several things into account. This graphic might be helpful...
Of particular concern is "Economic Standing", which includes personal solvency and a stable source of income linked to the applicant's economy. Another important criterion is "Social Standing", which means lots of ties to the applicant's society and by extension, country. These and the other criteria are used to predict the probable outcome of issuing a visa to the applicant.
They will also use a 'risk index' to scale up or scale down the result. The risk index is a composite figure over all the visas issued to a given nationality (in your case Sri Lanka) and incorporates their estimate of likelihood that an individual will abscond or in some other way breach the visa's conditions.
None of the criteria shown in the graphic are show-stoppers in their own right, but a problem in more than one area coupled with a 'risky nationality' will often result in a refusal. For example, a refusal of “Justification for the purpose and conditions of the intended stay was not provided” often means there was a hit on "Premise of the Visit" and if this is combined with a hit in any other area along with a moderately high risk index, the applicant will get a refusal.
For another example, a refusal citing "The information submitted regarding the justification for the purpose and conditions of the intended stay was not reliable" usually means there was a hit on "Quality of Evidence" (trying to provide a personal attestation instead of hard evidence or otherwise using poor evidence). If this is combined with another hit and the applicant draws even a low to medium risk index, it almost certainly results in an unfavourable risk assessment.
Schengen refusals do not disclose the exact reasons for this type of refusal and because of this using the internet is a really poor way to make a better application. To get a definitive answer, you would have to list out all of these things in full public view so that people can be properly informed to give assistance. It's better to arrange a consultation with a legal professional who operates a practice area in Schengen visas who can examine your stuff behind a confidential veil. Making a fresh application with the same stuff will invariably cause another refusal and then matters will be worse.
And on the other side, just bombarding them with lots more stuff in an appeal (or even fresh application) may not hit the target at all. In some cases, just throwing in more and more stuff for evidence (or making promises to them about your intentions) may be taken as a sign that you do not understand the visa and hence you do not understand its terms and hence you are likely to violate the terms and conditions and hence you become a 'risky applicant'.
