I always use the same spice mix for dal. Instead of taking 1-2 tsp of each, I would rather keep a special container for the dal of the ready made mix in the proportions I want.
Is there any reason not to do so? Shelf-life, aggregation, ..?
Assuming you're using spices which are all dried and ground, there should be no problem.
In the middle-east, there are always several spice mixtures available in shops. The most famous of which are Ras-al-Hanout and Baharat. These are spice mixtures sold as pre-mixed combinations by the shopkeeper, who is usually the one who grinds the spices.
I'm aware of three reasons that you might not want to do so:
If you only tend to cook one dish or you leave some of each spice in reserve, the first one isn't really a problem.
The second one is a function of the dish being prepared, which I'll assume isn't the case here.
The last one is less of a problem with ground spices, but can be significant for whole seeds -- as you jostle the container with use, some spices will tend to float to the top, while others sink to the bottom. The end result is that when you go take your measure of mixed spices, it might not have the same proportion as what you had expected.
You can partially mitigate the issue by taking your container and rolling it around on its side for a bit before righting the container and taking your measurement. It's not perfect, but it will help in re-distributing the spices.
If there are certain spices that would be a major problem if they weren't help in balance (eg, ground hot peppers), you can blend everything else and leave that one item separate.
You may want to keep them separate for shelf-life reasons. If you combine them, the shelf life of the mixture will be limited by the freshness of the least-fresh spice you mixed into it. Different spices' flavors also degrade at different rates, though generally you don't have to worry about the flavor of dried, ground spices degrading for at least 6 months. Depending on how much of this mixture you use, these things may not be a factor.
For some here there might be two other reasons: Practice, and variation. Mixing spices as part of the prep trains your memory, and sometimes helps you understand the mixtures, and there is a learning effect both from getting the balance slightly wrong and from getting it right in a subtly different way. This can also prevent a dish from getting boring if you make it a lot.