Why we were defensive ?

Indian empires throughout the ancient and medieval ages were fiercely territorial. And when I say this, I am not talking about the Indo-Greeks, Sakas, Kushans or the early Islamic invaders. They were not inherently Indian and were in fact aggressors on our land which was foreign to them.

To have an expansionary strategy needs to have the will power of the empire, desire of the rulers, a compelling reason which necessitates the development of an aggressive outlook. The Islamic horde came from the Central Asia which was a region of constant strife due to scarcity of resources. This compelled likes of Ghurid , Ghaznavid and later even Babar to look towards India to conquer it for its abundant resources and wealth. The later set up the Mughal empire which ruled India for about 200 years. 

But the fact remains why none of the empires which established in India like the Maurya's, Guptas and later the Mughals did not opt for an expansionary strategy like the European empires. The fact of the matter remains that they were well entrenched in Mutiple aspects. The most important aspect is the subcontinents Geography. There were sea and ocean on the three sides, Himalayas to the North to Northeast. Though there was the Khyber Pass through which the invaders descended in the Northwest. Yet it became absorbed once Mughals established their hegemony with the Baloch areas and Western Afghanistan becoming buffer areas between the Persian empire and the Mughals. This Geography forced the rulers to be less expansive in their approach. But yet, even European empires had to cover oceans to reach new shores like the Spanish Empire did to the Americas. So next comes the point that there enough resources to be satisfied with. India was already an export driven economy for a long time. There was abundance of natural resources and food as well. Also, the local empires were busy organizing their feudatories and vassals within the Indian Geographic realm to keep the supply lines and trade revenue intact. There was enough to focus on home than to look outside. Even the navies of Marathas were mostly coastal navies who were there to keep check on the pirates looting the ships in the coastal areas.

But yet there have been instances in the past when we had to adopt and expansionary shift. Cholas raided the Southeast Asian ports of hostile Sri Vijayan empire to establish and secure their supply routes to China. This was not a colonial ambition and was purely for economic reasons. It's no wonder that we had healthy trade relations with the rulers there which is pretty much visible in the cultural sphere even today. 

We could have been very well aggressive or offensive by choice, but the unique mix of factors allowed the various Indian rulers adopt a mindset which was pretty much in sync with securing and growing what was already there. Even come to think of it, Mughals who were contemporary of Ottomans in Turkey never felt the need to colonize Africa or push their Armies through Burma and Southeast Asia or adopt a Naval strategy to look for new colonies.