Introduction:
The Diels-Alder reaction is a powerful synthetic tool used to construct cyclic compounds. It involves the reaction between a conjugated diene and a dienophile to form a cyclic product known as the cycloadduct. In most cases, the major product obtained in the Diels-Alder reaction is the endo-adduct. This preference for the endo product can be explained by several factors.

Explanation:
1. Secondary orbital interactions:
The endo-adduct is favored due to the presence of secondary orbital interactions between the diene and dienophile. These interactions occur between the non-bonding electron pair on the diene and the empty orbital on the dienophile. This stabilizes the transition state leading to the formation of the endo product. The interaction is more favorable in the endo position compared to the exo position, leading to the preference for the endo-adduct.

2. Steric hindrance:
Steric hindrance also plays a role in determining the regioselectivity of the Diels-Alder reaction. The endo-product is often favored due to the lower steric hindrance in the transition state leading to its formation. The exo-product is hindered by the presence of bulky substituents that can lead to steric clashes, making the formation of the endo-product more favorable.

3. Stability of the product:
The endo-adduct is generally more stable than the exo-adduct due to the spatial arrangement of substituents. The endo-product often has substituents arranged in a more favorable conformation, leading to lower steric strain and increased stability. This stability contributes to the preference for the endo product in the Diels-Alder reaction.

4. Faster rate of formation:
In some cases, the endo-product may also be favored due to the faster rate of formation. The transition state leading to the endo-product may have a lower activation energy compared to the transition state leading to the exo-product. This kinetic preference can be attributed to a combination of factors, including orbital interactions and steric effects.

Conclusion:
In conclusion, the preference for the endo-adduct in the Diels-Alder reaction can be attributed to secondary orbital interactions, steric hindrance, stability of the product, and sometimes a faster rate of formation. These factors work together to determine the regioselectivity of the reaction, leading to the predominant formation of the endo product.