"Stronger" in terms of flavour delivery really amounts to a couple things: freshness, and preparation.
Strength of steeped tea comes from the ability of the tea leaf to absorb water, saturate and then infuse the water with flavinoids. With coffee we describe this as extraction, in teas we're dealing with infusion, really this is the same thing... transfer of flavour from leaf, into water.
Tea which has been on a shelf for a long time, and which may have been exposed to light, air, moisture and temperature variation (either in a tea bag, or loose) will have lost most of its volatile flavours and aromatics. This is due to oxidation. The tea is stale.
So if you bought a commercially prepare tea, that has been sitting on a retailer's shelf for a long time, it's likely to be less fresh than a tea purchased from a boutique tea retailer (or online store) which turns over their volumes readily and regularly.
If your tea is fresh, the next thing that will deliver "strength" is how you prepare it. Here are tea brewing basics:
1. Use freshly drawn water – discard old water from your kettle. If the quality of your water is questionable, use spring water.
2. Bring water to a boil for black teas, oolongs and herbals (tisanes), 80C for white and green teas.
3. Measure one teaspoon measurement per cup of tea into a large infusor – it is important that your infusor is large enough to allow the leaves to unfurl and fully infuse their flavours. You should always refer to the brewing instructions of your specific tea for measurements as some teas will vary.
4. Pre-heat your teapot – this will keep your tea warmer for a longer amount of time as the teapot won’t steal the heat away from your tea.
5. Pour water OVER the tea leaves – allow to infuse depending on the infusion time of the tea you are brewing.
6. Remove tea infusor from water – sit back and enjoy.